feat(math): add support for some math functions (#483)

pallas-math/src/lib.rs

@@ -1 +1,14 @@
 pub mod math;
+
+// Ensure only one of `gmp` or `num` is enabled, not both.
+#[cfg(all(feature = "gmp", feature = "num"))]
+compile_error!("Features `gmp` and `num` are mutually exclusive.");
+
+#[cfg(all(not(feature = "gmp"), not(feature = "num")))]
+compile_error!("One of the features `gmp` or `num` must be enabled.");
+
+#[cfg(feature = "gmp")]
+pub mod math_gmp;
+
+#[cfg(feature = "num")]
+pub mod math_num;