perf(types): 3x speedup MakePartSet (#3117)

[ValarDragon]

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

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• Tests written/updated
• Changelog entry added in .changelog (we use unclog to manage our changelog)
• Updated relevant documentation (docs/ or spec/) and code comments
• Title follows the Conventional Commits spec

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PR checklist

• Tests written/updated
• Changelog entry added in .changelog (we use unclog to manage our changelog)
• Updated relevant documentation (docs/ or spec/) and code comments