Change internals of PublicKey to store both compressed Edwards Y and decompressed point forms

[isislovecruft]

Since the type is currently #[repr(C)] this will cause issues for anyone who was passing it over an FFI boundary directly (i.e. without first using the serde serialisation or calling to_bytes(), etc.). However, this will give us a speedup on non-batch verification as it avoids the point decompression.

[hdevalence]

It might be good to remove the #[repr(C)] and provide byte-oriented functions for FFI, so that the Rust types aren't encumbered by FFI considerations at all and can maintain whatever invariants they want.

[isislovecruft]

This was implemented in #62 and will be available in 1.0.0-pre.1. It gives roughly a 35% speed up on key reuse for signature verification (batch or otherwise).