Flesh out `Format`

[dekarrin]

This type adds support for describing all of the things in a data stream header as described in #46 .

• Fill out stub for type Format struct, with most fields unexported.
• Create a new function, V1(), which accepts no parameters and returns a Format that represents the old style data format, compatible with reading non-headered rezi data streams all the way back to Rezi v1.x.y.
• Create a new function, V2(), which accepts parameters related to compression and returns a Format that represents the new style data format.
• Add Format.MarshalBinary for encoding it as bytes. It must return empty bytes for V1 formats.
• Add Format.UnmarshalBinary for decoding it from bytes. It must accept empty bytes and return a V1 if so.

This should be a new type that is not created by hand - function V2() creates a new format in the new one and accepts any parameters that would be given. Function V1() is how a Format representing the old format is obtained and will be used by default by any routines that are modified to accept an optional Format.

[dekarrin]

Noted that this will already exist as a stub from work on #11