Use RawVec for `Int`s memory management.

[huonw]

This abstracts out handling the allocator with alignment/sizes etc, and has most interesting operations predefined.

[huonw]

The use of reserve rather than reserve_exact means things can be a little over-allocated, but this isn't necessarily wrong?

[Aatch]

It looks like reserve allocates for double the space you actually ask for. As the only operation to grow the size rapidly is multiplication, and that needs a separate allocation anyway, I'd rather not over-allocate quite so much.

[huonw]

Ok, switched to reserve_exact.

[Aatch]

Sweet!