Improve pool implementation

Improve pool implementation

Move virtual memory failures into the `virtual_reserve` and
`virtual_commit` signatures so we can give better assert messages on
failure.

Handle growth more accurately, allowing us to use the entire table
capacity before asserting.

Reserve a bit in the generation counter to keep track of whether a slot
is full by incrementing the counter on both allocation and deallocation.
This means exported handles can only ever have an odd generation
counter.

Assert when given a handle that has an invalid generation counter
(where the counter implies it would be pointing to an empty slot).

Both above changes together means it's no longer possible to create a
reference to an uninitialized slot, even when manually messing with the
handle or mixing handles between different pools.