Hashing improvements

[hoxxep]

As part of investigating whether rust-prometheus would benefit from rapidhash, I noticed a few improvements that could be made:

• The MetricVecCore::children hashmap uses keys that are hash values, so introducing NoHashHasher to avoid re-hashing this hash value improves performance.
• Allow with and get_metric_with to accept hashmaps with custom hashers.
• Introduce criterion's black_box in some hashmaps to reduce over-optimisation in microbenchmarks. Could be applied to further benchmarks.

Some of the counter.rs benchmarks are including hashmap initialisation and destruction. They are also using fixed values and small keys/values, which means benchmarks were unclear over whether FNV or rapidhash are beneficial (hash clashes, etc). Slightly longer key/value lengths are likely to benefit from rapidhash.

[ti-chi-bot]

Welcome @hoxxep! It looks like this is your first PR to tikv/rust-prometheus 🎉