layers: Distinguish never-bound from un-bound memory cases

layers: Distinguish never-bound from un-bound memory cases

Fixes #964

Added special MEMORY_UNBOUND handle (0xF..FE) that indicates that memory
bound to an object has been freed. When attempting to bind memory or
checking for bound memory, distinguish the never-bound case from the
memory un-bound case.

For sparse binding case allow for memory to be re-bound.

Update tests to account for new error messages.

There's a sliver of exposure here if an actual memory handle is MEMORY_UNBOUND.
We could remove that exposure by never having unique_objects return MEMORY_UNBOUND
as a handle. I believe the exposure is small enough that we don't need to do that,
but am open to other opinions.