Split decoding loop

[fintelia]

This PR splits the main decoding loop into a "fast" loop that avoids most bounds checks, and a "careful" loop to handle the remaining input. The fast decoding loop also avoids redundant table lookups and generally does table lookups before refilling the bit buffer. These changes were inspired by the libdeflate library which uses a similar approach.

I measured a roughly 10% end-to-end speedup on PNG decoding from this change.