bit: Add test cases. Copy without modification from https://github.com/eaburns/bit/blob/master/bit_test.go, MIT license.
bit: Print additional test case details.
bit: Optimize!

benchmark                old ns/op     new ns/op     delta
BenchmarkReadAlign1      118188        101941        -13.75%
BenchmarkReadAlign32     73882         60544         -18.05%
BenchmarkReadAlign64     55018         44432         -19.24%

benchmark                old MB/s     new MB/s     speedup
BenchmarkReadAlign1      34.66        40.18        1.16x
BenchmarkReadAlign32     55.44        67.65        1.22x
BenchmarkReadAlign64     74.45        92.18        1.24x