Adds a trait for crypto, porting EC first (#606)

* Adds a trait for crypto, porting EC first

* Moves crypto implementation next to its trait

* Renames constants and types

libraries/crypto/src/ecdsa.rs

@@ -276,7 +276,6 @@ impl PubKey {
         ExponentP256::modn(u.to_int()) == *sign.r.as_exponent()
     }
 
-    #[cfg(feature = "std")]
     pub fn verify_vartime<H>(&self, msg: &[u8], sign: &Signature) -> bool
     where
         H: Hash256,