bzip2: improve performance
Improve performance of move-to-front by using cache-friendly
copies instead of doubly-linked list. Simplify so that the
underlying slice is the object. Remove the n=0 special case,
which was actually slower with the copy approach.
benchmark old ns/op new ns/op delta
BenchmarkDecodeDigits
26429714 23859699 -9.72%
BenchmarkDecodeTwain
76684510 67591946 -11.86%
benchmark old MB/s new MB/s speedup
BenchmarkDecodeDigits 1.63 1.81 1.11x
BenchmarkDecodeTwain 1.63 1.85 1.13x
Updates #6754.
LGTM=adg, agl, josharian
R=adg, agl, josharian
CC=golang-codereviews
https://golang.org/cl/
131840043