Use criterion instead of unstable libtest for benches

[j-g00da]

Resolves #20

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[j-g00da]

1. bench.rs moved to benches:

It is not possible to include benchmarks in code in the src/ directory as one might with the regular benchmark harness.

2. Making pb_test::gen public:

It is not possible to benchmark non-pub functions. External benchmarks, including those using Criterion.rs, are compiled as a separate crate, and non-pub functions are not visible to the benchmarks.

https://bheisler.github.io/criterion.rs/book/user_guide/known_limitations.html