Moves our projection handling code into a common file, and avoids the use of a
general mplace-based fallback function by have more specialized implementations.
mplace_index (and the other slice-related functions) could be more efficient by
copy-pasting the body of operand_index. Or we could do some trait magic to share
the code between them. But for now this is probably fine.
We now have an infallible function that also tells us which kind of allocation we are talking about.
Also we do longer have to distinguish between data and function allocations for liveness.
