RawVec should round capacity to jemalloc bucket size

[hsivonen]

By inspection of source code, it seems that RawVec is unaware of jemalloc bucket sizes. This is unfortunate, because it's wasteful to reallocate when the required capacity would fit within the current jemalloc allocation but RawVec just doesn't know.

As far as abstraction leakage goes, it would be better to let the leakage stop at RawVec than to have higher-level code try to second-guess RawVec with assumptions about the underlying allocator.

I suggest that the allocator crate provide a rounding function for rounding up to the largest allocation size that would consume as much space in the underlying allocator as the requested size and RawVec use it to round up capacity. For jemalloc, the rounding function would round to the jemalloc bucket size.

[nagisa]

Duplicate of #29931 and #32075

[Mark-Simulacrum]

Closing in favor of #29931.