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.

crypto/prime-field/tests/5mod8.rs

@@ -0,0 +1,6 @@
+prime_field::odd_prime_field!(
+  Scalar,
+  "0x1000000000000000000000000000000014def9dea2f79cd65812631a5cf5d3ed",
+  "02",
+  false
+);