bitv: add larger tests, better benchmarks & remove dead code.

There were no tests for iteration etc. with more than 5 elements,
i.e. not even going beyond a single word. This situation is rectified.

Also, the only benchmarks for `set` were with a constant bit value,
which was not indicative of every situation, due to inlining & branch
removal. This adds a benchmark at the other end of the spectrum: random
input.
auto merge of #16961 : huonw/rust/bitv-twiddle, r=alexcrichton

bitv: add larger tests, better benchmarks & remove dead code.

There were no tests for iteration etc. with more than 5 elements,
i.e. not even going beyond a single word. This situation is rectified.

Also, the only benchmarks for `set` were with a constant bit value,
which was not indicative of every situation, due to inlining & branch
removal. This adds a benchmark at the other end of the spectrum: random
input.