Use Box<[Word]> for word storage in DenseBitSet

[Zalathar]

Since DenseBitSet has a fixed domain size, it doesn't need the capacity field of Vec. This shrinks the inline-size of DenseBitSet from 32 bytes to 24 bytes.

The main reason this wasn't done earlier is that GrowableBitSet does require resizing, and was using DenseBitSet as its internal representation.

Thus, most of this PR is separating the two bitset implementations, so that DenseBitSet is free to use a more appropriate storage type.

[Zalathar]

cc @panstromek

@bors try @rust-timer queue

[Zalathar]

For future work, we could potentially shrink DenseBitSet down to 16 bytes by only storing (pointer, domain_size) and inferring allocation length from the domain size. But that would require unsafe shenanigans that the implementation isn't ready for.

[rust-bors]

☀️ Try build successful (CI)
Build commit: cfa61c7 (cfa61c735d872ca7c526a0a3a312336e026fc444)
Base parent: 17fd5b8 (17fd5b8a37b6667b6cc137f3cc35f09759768a3b)

[panstromek]

Nice, I was going to try this one too, it should help cases when we store DenseBitSets in Vecs, like SparseBitMatrix. For those, it would be even better to store domain_size out of the datatructure completely and maybe store the data inline. That's part of the motivation for why I'm experimenting with making the DenseBitSet generic over storage (one experiment for this pattern is #161584) to make it possible to make these decisions per usage (and why I think your raw PR would make that easier, too).

As I was looking at the usages of these (DenseBitSets in Vecs, SparseBitMatrix), I found that they are often not quite optimally used anyway, so it might be better to just eliminate them, so I did one in #161850 and I think we can eliminate other ones too. That might reduce the impact of this PR, but we still probably have a ton of BitSets in other places where it'll help.

[rust-timer]

Finished benchmarking commit (cfa61c7): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.8%, -0.2%]	4
Improvements ✅ (secondary)	-0.2%	[-0.4%, -0.0%]	10
All ❌✅ (primary)	-0.5%	[-0.8%, -0.2%]	4

Max RSS (memory usage)

Results (primary 2.6%, secondary 1.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	2.7%	[0.8%, 6.3%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

Cycles

Results (primary 2.3%, secondary 4.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	9.7%	[2.1%, 14.5%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.3%	[-4.3%, -4.3%]	2
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	1

Binary size

Results (secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-	-	0

Bootstrap: 474.378s -> 479.269s (1.03%)
Artifact size: 402.85 MiB -> 402.83 MiB (-0.01%)

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

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