Switch 32-bit platforms to using 64-bit "pointers"

Switch 32-bit platforms to using 64-bit "pointers"

This much more cleanly fixes the issue described in the previous
commit by simply using 64-bit integers to represent pointers on
32-bit platforms, letting us store the required flag bit in the top
32-bits that are always free.

It does, however, imply changing the entire call semantics for
TypeScript to use BigInts for pointers, rather than numbers.