mash: optimize mash similarity function, add benchmark

[matiasinsaurralde]

Changes in this PR

• Optimizes the mash similarity function: removed sort.Search usage, merged the early exit conditions and rewrote the sketch traversing logic.
• Adds simple mash.Distance benchmark.

Why are you making these changes?

• General optimization:

  
  % benchcmp old.txt new.txt 
  benchmark           old ns/op     new ns/op     delta
  BenchmarkMash-8     118           9.71          -91.75%
  BenchmarkMash-8     113           9.72          -91.43%
  BenchmarkMash-8     113           9.71          -91.43%

benchmark old allocs new allocs delta BenchmarkMash-8 0 0 +0.00% BenchmarkMash-8 0 0 +0.00% BenchmarkMash-8 0 0 +0.00%

benchmark old bytes new bytes delta BenchmarkMash-8 0 0 +0.00% BenchmarkMash-8 0 0 +0.00% BenchmarkMash-8 0 0 +0.00%

### Are any changes breaking? (IMPORTANT)
No - tests look fine

## Pre-merge checklist
*All of these must be satisfied before this PR is considered
ready for merging. Mergeable PRs will be prioritized for review.*

* [ ] New packages/exported functions have docstrings.
* [x] New/changed functionality is thoroughly tested.
* [ ] New/changed functionality has a function giving an example of its usage in the associated test file. See `primers/primers_test.go` for what this might look like.
* [ ] Changes are documented in `CHANGELOG.md` in the `[Unreleased]` section.
* [x] All code is properly formatted and linted.
* [x] The PR template is filled out.