Group trait overhaul (#52)

* Decouple element from `Group`

* Change `SUITE_ID` to `u16` and rework `get_context_string()`

* Rework scalar de-serialization

* Rename `Group` methods

- `random_nonzero_scalar` -> `random_scalar`
- `scalar_as_bytes` -> `serialize_scalar`
- `scalar_invert` -> `invert_scalar`

* Rework element de-serialization

* Rename and remove `Group` methods

`to_arr` -> `serialize_elem`
`base_point` -> `base_elem`
`is_identity` -> removed
`identity` -> `identity_elem`
`zero_scalar` -> hidden behind `cfg(test)`

* Sort `Group` methods

* Rework `expand_message_xmd` and remove utility

* Improve P256 `hash_to_scalar`

src/error.rs

@@ -34,8 +34,8 @@ pub enum Error {
     ProofVerificationError,
     /// Encountered insufficient bytes when attempting to deserialize
     SizeError,
-    /// Encountered a zero scalar
-    ZeroScalarError,
+    /// Encountered an invalid scalar
+    ScalarError,
 }
 
 #[cfg(feature = "std")]