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This PR adds some benchmarks, and significantly speeds up
types.MakePartSet, and Partset.AddPart. (Used by the block proposer, and
every consensus instance) It does so by doing two things:
- Saving mutexes on the newly created bit array, by defaulting every
value to True (rather than setting it in a loop that goes through a
mutex)
- Uses the same hash object throughout, and avoids an extra copy of
every leaf. (main speedup)

I do the same hash optimization for proof.Verify, which is used in the
add block part codepath for both the proposer and every full node.

New:
```
BenchmarkMakePartSet/nParts=1-12         	   38616	     29817 ns/op	     568 B/op	      12 allocs/op
BenchmarkMakePartSet/nParts=2-12         	   19888	     59866 ns/op	    1000 B/op	      22 allocs/op
BenchmarkMakePartSet/nParts=3-12         	   12979	     95691 ns/op	    1528 B/op	      33 allocs/op
BenchmarkMakePartSet/nParts=4-12         	    8688	    128192 ns/op	    2024 B/op	      44 allocs/op
BenchmarkMakePartSet/nParts=5-12         	    7308	    155224 ns/op	    2888 B/op	      57 allocs/op
```

Old:
```
BenchmarkMakePartSet/nParts=1-12         	   16647	    106545 ns/op	   74169 B/op	      12 allocs/op
BenchmarkMakePartSet/nParts=2-12         	   10000	    106361 ns/op	  148329 B/op	      23 allocs/op
BenchmarkMakePartSet/nParts=3-12         	    6992	    337644 ns/op	  222587 B/op	      35 allocs/op
BenchmarkMakePartSet/nParts=4-12         	    3488	    480109 ns/op	  296811 B/op	      47 allocs/op
BenchmarkMakePartSet/nParts=5-12         	    2228	    557768 ns/op	  371404 B/op	      61 allocs/op
```

System wide, this is definitely not our issue (looks like roughly .1ms
per blockpart), but still definitely useful time to remove

---

- [x] Tests written/updated
- [x] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [x] Updated relevant documentation (`docs/` or `spec/`) and code
comments
- [x] Title follows the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) spec

(cherry picked from commit 7b1c1f8)