Add benchmarks.
Remove allocations in bit.buffer.

This improves performance significantly:

benchmark               old ns/op    new ns/op    delta
BenchmarkReadAlign1        118644        71504  -39.73%
BenchmarkReadAlign32        80302        47901  -40.35%
BenchmarkReadAlign64        54746        30309  -44.64%

benchmark                old MB/s     new MB/s  speedup
BenchmarkReadAlign1         34.52        57.28    1.66x
BenchmarkReadAlign32        51.01        85.51    1.68x
BenchmarkReadAlign64        74.82       135.14    1.81x