Make `SmallVector` more general-purpose

[nnethercote]

SmallVector is a very useful type. It can avoid lots of heap allocations when used on hot paths, which can give nice speed-ups. I have used it in multiple places, and I have more changes in the pipeline that will use it.

But it's not quite as general-purpose as it could be. Ideas for improvement:

• Rename it as SmallVec, to mirror Vec.
• Move it from libsyntax to librustc_data_structures.
• Increase the number of elements that can be present without requiring heap allocation from 1 to 2 or 4. (Because 1 covers a lot of cases, but 2 or 4 would cover more.)
• Tweak the API to better match Vec. E.g. change zero() to new().

[Mark-Simulacrum]

I've started some part of this work in implementing a SmallVec8, with a proposed rename to ArrayVec and the accompanying possibly-heap variant AccumulateVec, which should land with #37270, but the ETA on that is not well-defined.

[Ms2ger]

I wonder if there's a way to use the existing rust-smallvec crate yet.