compress/flate: extract LZ77 dictionary logic into seperate struct
The LZ77 portion of DEFLATE is relatively self-contained. For the
decompression side of things, we extract this logic out for the
following reasons:
* It is easier to test just the LZ77 portion of the logic.
* It reduces the noise in the inflate.go
Also, we adjust the way that callbacks are handled in the inflate.
Instead of using functions to abstract the logical componets of
huffmanBlock(), use goto statements to jump between the necessary
sections. This is faster since it avoids a function call and is
arguably more readable.