Improve bloom microbenchmarks.
Improve bloom microbenchmarks.

The current code is mostly bechmarking Rust's default hash routines for
integers, which is not interesting to us. We add generating function
that jumps around "enough" and also add benchmarks for clear().

It's hard to read too much into the numbers because we can't reliably
simulate L1/L2 cache behavior with cargo bench, but the results show
about 40ns for clear() about 2ns for insert, and about 1.5ns for
contains/remove. This informs our thinking in the next patch.

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