bitv: make benchmarks always return a value

This makes sure that the benchmarked code does not get optimized away.
Also fixed a typo.

Fixes #12118.
arena: fix a typo
bitv: make sure benchmarks run long enough

Previously they were too short (less than 10 ns), so the benchmarker
could not resolve them meaningfully. Now they should run in the order of
100 ns.
auto merge of #16651 : vks/rust/fix-bitv-bench, r=alexcrichton

Fixes #12118.

(I sneaked in an unrelated one-character whitespace fix I spotted while reviewing some benchmarks, if that is not okay, I can create a separate pull request for that.)