Add more transparency into some EC structures. (#641)

This adds the ability to create ECDH keys from raw bytes and export
signatures as raw bytes.

libraries/opensk/src/api/crypto/ecdsa.rs

@@ -73,7 +73,6 @@ pub trait Signature: Sized {
     fn from_slice(bytes: &[u8; EC_SIGNATURE_SIZE]) -> Option<Self>;
 
     /// Writes the signature bytes into the passed in parameter.
-    #[cfg(feature = "std")]
     fn to_slice(&self, bytes: &mut [u8; EC_SIGNATURE_SIZE]);
 
     /// Encodes the signatures as ASN1 DER.

libraries/opensk/src/api/crypto/software_crypto.rs

@@ -169,7 +169,6 @@ impl ecdsa::Signature for SoftwareEcdsaSignature {
         crypto::ecdsa::Signature::from_bytes(bytes).map(|s| SoftwareEcdsaSignature { signature: s })
     }
 
-    #[cfg(feature = "std")]
     fn to_slice(&self, bytes: &mut [u8; EC_SIGNATURE_SIZE]) {
         self.signature.to_bytes(bytes);
     }