Auto merge of #37270 - Mark-Simulacrum:smallvec-optimized-arenas, r=eddyb

Add ArrayVec and AccumulateVec to reduce heap allocations during interning of slices

Updates `mk_tup`, `mk_type_list`, and `mk_substs` to allow interning directly from iterators. The previous PR, #37220, changed some of the calls to pass a borrowed slice from `Vec` instead of directly passing the iterator, and these changes further optimize that to avoid the allocation entirely.

This change yields 50% less malloc calls in [some cases](https://pastebin.mozilla.org/8921686). It also yields decent, though not amazing, performance improvements:
```
futures-rs-test  4.091s vs  4.021s --> 1.017x faster (variance: 1.004x, 1.004x)
helloworld       0.219s vs  0.220s --> 0.993x faster (variance: 1.010x, 1.018x)
html5ever-2016-  3.805s vs  3.736s --> 1.018x faster (variance: 1.003x, 1.009x)
hyper.0.5.0      4.609s vs  4.571s --> 1.008x faster (variance: 1.015x, 1.017x)
inflate-0.1.0    3.864s vs  3.883s --> 0.995x faster (variance: 1.232x, 1.005x)
issue-32062-equ  0.309s vs  0.299s --> 1.033x faster (variance: 1.014x, 1.003x)
issue-32278-big  1.614s vs  1.594s --> 1.013x faster (variance: 1.007x, 1.004x)
jld-day15-parse  1.390s vs  1.326s --> 1.049x faster (variance: 1.006x, 1.009x)
piston-image-0. 10.930s vs 10.675s --> 1.024x faster (variance: 1.006x, 1.010x)
reddit-stress    2.302s vs  2.261s --> 1.019x faster (variance: 1.010x, 1.026x)
regex.0.1.30     2.250s vs  2.240s --> 1.005x faster (variance: 1.087x, 1.011x)
rust-encoding-0  1.895s vs  1.887s --> 1.005x faster (variance: 1.005x, 1.018x)
syntex-0.42.2   29.045s vs 28.663s --> 1.013x faster (variance: 1.004x, 1.006x)
syntex-0.42.2-i 13.925s vs 13.868s --> 1.004x faster (variance: 1.022x, 1.007x)
```

We implement a small-size optimized vector, intended to be used primarily for collection of presumed to be short iterators. This vector cannot be "upsized/reallocated" into a heap-allocated vector, since that would require (slow) branching logic, but during the initial collection from an iterator heap-allocation is possible.

We make the new `AccumulateVec` and `ArrayVec` generic over implementors of the `Array` trait, of which there is currently one, `[T; 8]`. In the future, this is likely to expand to other values of N.

Huge thanks to @nnethercote for collecting the performance and other statistics mentioned above.

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