This seriously improves the serialization/deserialization performance for
several reasons:
* no time spent in `binary` reflection
* no memory allocations being made on every read/write
* uses fast ReadBytes everywhere it's appropriate
It also makes Fixed8 Serializable just for convenience.
It's mostly used for Serializable and in other cases where one needs to
estimate binary-encoded size of the stucture. This also simplifies future
removal of the Size() from Serializable.
