chore: only implement Error with feature "std"

`core::error` needs an MSRV of 1.81,
which is a bit too new for this crate

src/distribution/beta.rs

@@ -43,6 +43,7 @@ impl core::fmt::Display for BetaError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for BetaError {}
 
 impl Beta {

src/distribution/binomial.rs

@@ -41,6 +41,7 @@ impl core::fmt::Display for BinomialError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for BinomialError {}
 
 impl Binomial {

src/distribution/categorical.rs

@@ -53,6 +53,7 @@ impl core::fmt::Display for CategoricalError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for CategoricalError {}
 
 impl Categorical {

src/distribution/cauchy.rs

@@ -42,6 +42,7 @@ impl core::fmt::Display for CauchyError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for CauchyError {}
 
 impl Cauchy {

src/distribution/chi.rs

@@ -42,6 +42,7 @@ impl core::fmt::Display for ChiError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for ChiError {}
 
 impl Chi {

src/distribution/dirac.rs

@@ -33,6 +33,7 @@ impl core::fmt::Display for DiracError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for DiracError {}
 
 impl Dirac {

src/distribution/dirichlet.rs

@@ -54,6 +54,7 @@ impl core::fmt::Display for DirichletError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for DirichletError {}
 
 impl Dirichlet<Dyn> {

src/distribution/discrete_uniform.rs

@@ -38,6 +38,7 @@ impl core::fmt::Display for DiscreteUniformError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for DiscreteUniformError {}
 
 impl DiscreteUniform {

src/distribution/exponential.rs

@@ -40,6 +40,7 @@ impl core::fmt::Display for ExpError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for ExpError {}
 
 impl Exp {

src/distribution/fisher_snedecor.rs

@@ -49,6 +49,7 @@ impl core::fmt::Display for FisherSnedecorError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for FisherSnedecorError {}
 
 impl FisherSnedecor {

src/distribution/gamma.rs

@@ -48,6 +48,7 @@ impl core::fmt::Display for GammaError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for GammaError {}
 
 impl Gamma {

src/distribution/geometric.rs

@@ -39,6 +39,7 @@ impl core::fmt::Display for GeometricError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for GeometricError {}
 
 impl Geometric {

src/distribution/gumbel.rs

@@ -45,6 +45,7 @@ impl core::fmt::Display for GumbelError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for GumbelError {}
 
 impl Gumbel {

src/distribution/hypergeometric.rs

@@ -48,6 +48,7 @@ impl core::fmt::Display for HypergeometricError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for HypergeometricError {}
 
 impl Hypergeometric {

src/distribution/inverse_gamma.rs

@@ -49,6 +49,7 @@ impl core::fmt::Display for InverseGammaError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for InverseGammaError {}
 
 impl InverseGamma {

src/distribution/laplace.rs

@@ -42,6 +42,7 @@ impl core::fmt::Display for LaplaceError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for LaplaceError {}
 
 impl Laplace {

src/distribution/log_normal.rs

@@ -46,6 +46,7 @@ impl core::fmt::Display for LogNormalError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for LogNormalError {}
 
 impl LogNormal {

src/distribution/multinomial.rs

@@ -59,6 +59,7 @@ impl core::fmt::Display for MultinomialError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for MultinomialError {}
 
 impl Multinomial<Dyn> {

src/distribution/multivariate_normal.rs

@@ -135,6 +135,7 @@ impl core::fmt::Display for MultivariateNormalError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for MultivariateNormalError {}
 
 impl MultivariateNormal<Dyn> {

src/distribution/multivariate_students_t.rs

@@ -82,6 +82,7 @@ impl core::fmt::Display for MultivariateStudentError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for MultivariateStudentError {}
 
 impl MultivariateStudent<Dyn> {

src/distribution/negative_binomial.rs

@@ -60,6 +60,7 @@ impl core::fmt::Display for NegativeBinomialError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for NegativeBinomialError {}
 
 impl NegativeBinomial {

src/distribution/normal.rs

@@ -46,6 +46,7 @@ impl core::fmt::Display for NormalError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for NormalError {}
 
 impl Normal {

src/distribution/pareto.rs

@@ -43,6 +43,7 @@ impl core::fmt::Display for ParetoError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for ParetoError {}
 
 impl Pareto {

src/distribution/poisson.rs

@@ -39,6 +39,7 @@ impl core::fmt::Display for PoissonError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for PoissonError {}
 
 impl Poisson {

src/distribution/students_t.rs

@@ -51,6 +51,7 @@ impl core::fmt::Display for StudentsTError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for StudentsTError {}
 
 impl StudentsT {

src/distribution/triangular.rs

@@ -58,6 +58,7 @@ impl core::fmt::Display for TriangularError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for TriangularError {}
 
 impl Triangular {

src/distribution/uniform.rs

@@ -49,6 +49,7 @@ impl core::fmt::Display for UniformError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for UniformError {}
 
 impl Uniform {

src/distribution/weibull.rs

@@ -47,6 +47,7 @@ impl core::fmt::Display for WeibullError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for WeibullError {}
 
 impl Weibull {

src/function/beta.rs

@@ -30,6 +30,7 @@ impl core::fmt::Display for BetaFuncError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for BetaFuncError {}
 
 /// Computes the natural logarithm

src/function/gamma.rs

@@ -26,6 +26,7 @@ impl core::fmt::Display for GammaFuncError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for GammaFuncError {}
 
 /// Auxiliary variable when evaluating the `gamma_ln` function

src/stats_tests/fisher.rs

@@ -117,6 +117,7 @@ impl core::fmt::Display for FishersExactTestError {
     }
 }
 
+#[cfg(feature = "std")]
 impl std::error::Error for FishersExactTestError {}
 
 impl From<HypergeometricError> for FishersExactTestError {