Correct Quantity::new to be zero-cost for float storage types.

Use a default constant of `-0.0` to allow for floating point
optimizations. For a value, `v: Float`, adding `-0.0` is a no-op while
adding `0.0` will change the sign if `v` is `-0.0`. Resolves #143.

   v
 0.0 + -0.0 =  0.0
-0.0 +  0.0 =  0.0 # v + 0.0 != v
-0.0 + -0.0 = -0.0

src/quantity.rs

@@ -493,5 +493,14 @@ macro_rules! quantity {
     (@coefficient $factor:expr, $const:expr) => { $factor };
     (@coefficient $factor:expr) => { $factor };
     (@constant $factor:expr, $const:expr) => { $const };
-    (@constant $factor:expr) => { 0.0 };
+    // Use a default constant of `-0.0` to allow for floating point optimizations. For a value,
+    // `v: Float`, adding `-0.0` is a no-op while adding `0.0` will change the sign if `v` is
+    // `-0.0`.
+    //
+    //    v
+    //  0.0 +  0.0 =  0.0
+    //  0.0 + -0.0 =  0.0
+    // -0.0 +  0.0 =  0.0 # v + 0.0 != v
+    // -0.0 + -0.0 = -0.0
+    (@constant $factor:expr) => { -0.0 };
 }