Replace complex "safe" code with clearer "unsafe" code.

There's probably a better way to do this.
Remove unnecessary parentheses from `rotl!` macro body.
Add benchmarks for sizes [0, 1, 2, 4, ..., 2048, 4096, 65536] bytes.
Move tests into separate file; add Makefile to run them.
t
Merge branch 'master' into test-travis