Faster u32 parsing (#428)

This way removes a separate `State::Signature` variant and handles the
PNG signature via two `State::U32` states.  This is desirable because:

* This helps to reuse the performance gains for `State::U32`
  (i.e. avoid accumulating the signature byte-by-byte if the input
  buffer already has all its bytes)
* Avoiding separate code also has additional benefits:
    - Less pressure on the instructions cache
    - Ability to reuse branch prediction for `U32`-related code

The modified code already has reasonable test coverage via
http://www.schaik.com/pngsuite/#corrupted

Author: anforowicz