Allow overriding the u32 and u64 backend selection via features

Cargo.toml

@@ -63,3 +63,11 @@ alloc = ["zeroize/alloc"]
 fiat_backend = ["fiat-crypto"]
 # The SIMD backend uses parallel formulas, using either AVX2 or AVX512-IFMA.
 simd_backend = ["nightly", "packed_simd"]
+
+# Target backend overrides for fiat_backend
+fiat_64 = []
+fiat_32 = []
+
+# Target backend overrides for the default serial backend
+serial_64 = []
+serial_32 = []

src/backend/serial/mod.rs

@@ -21,7 +21,8 @@
 use cfg_if::cfg_if;
 
 cfg_if! {
-    if #[cfg(feature = "fiat_backend")] {
+
+    if #[cfg(any(feature = "fiat_backend", feature = "fiat_64", feature = "fiat_32"))] {
 
         #[cfg(all(feature = "fiat_64", feature = "fiat_32"))]
         compile_error!("Overrides fiat_64 or fiat_32 are mutually exclusive - cannot enable both.");