swift234

[Compression] Accelerate the code that decodes chars from var-length streams.

This change accelerates the code that decodes characters from bitstreams
(represented using APInts) that are encoded using variable length encoding. The
idea is simple: Extract the lowest 64 bit and decode as many characters as
possible before changing the bignum. Decoding words from a local variable is
much faster than changing big nums. This commit makes decompression twice as
fast as compression.