Start passing the user-data for the header down

Right now, just barfs if you try to use it (but not exposed to Perl yet
anyway).
Cosmetics
Abstraction for buffer allocation/deallocation

Includes functions for copying (shallowly) and swapping buffers.
Eat your own dog food: Use buffer alloc interface for snappy
Detect when the Snappy compression was net negative and back out

Made easy by the new buffer abstraction. Yay!