…alize`

- this saves an alllocation w.r.t the suggested approach by implementing `io::Write` on the input instance of `digest::Digest`,
- note that most instances of `digest::Digest` [already](https://gist.github.com/huitseeker/e827161413063e347ce5a496b66ff287) have an [`io::Write` instance](https://github.com/rustcrypto/hashes#hashing-readable-objects), but `CanonicalSerialize` consuming its `io::Write` argument prevents its usage,
- this hence implements `io::Write` on a [cheap newtype wrapper](https://rust-unofficial.github.io/patterns/patterns/behavioural/newtype.html)

Fixes arkworks-rs#86

- rename Hash -> CanonicalSerialize Ext according to [extension trait best practices](https://rust-lang.github.io/rfcs/0445-extension-trait-conventions.html#the-convention)
- add the same structure for hashing uncompressed bytes