Add prime-field crate

prime-field introduces a macro to generate a prime field, in its entitrety,
de-duplicating code across minimal-ed448, embedwards25519, and secq256k1.

tests/no-std/src/lib.rs

@@ -11,6 +11,7 @@ pub use multiexp;
 
 pub use schnorr_signatures;
 
+pub use prime_field;
 pub use secq256k1;
 pub use embedwards25519;