Immutably borrowing BumpVec?

[bluurryy]

We could implement a BumpVec that just immutably borrows the bump allocator. There is a stub implementation in src/vec.rs that we use for alloc_iter and alloc_fmt because the standard Vec produced really bad code for some reason.

An immutably borrowing BumpVec is pretty much the same as Vec. But it could implement conversion to FixedBumpVec and BumpBox<[T]> and back. MIN_ALIGN would also affect it unlike Vec.

If so we should probably rename the current BumpVec, MutBumpVec or something.