Use aligned loads for Range::find_first_of

Use aligned loads for Range::find_first_of

Summary:
Aligned loads are faster on some architectures. Let's refactor
qfind_first_of so that it uses aligned loads when possible.

Also modify the benchmarks to account for begin/end-of-string
logic.

Test Plan:
Tests pass. Updated benchmarks.  Generally seeing a 5-20%
speed-up, depending on the situation.

Reviewed By: philipp@fb.com

FB internal diff: D720369