This is a replacement for `FragmentedMapper`.  It does not use hash
tables and is lock-free when looking up or lazily initializing the slabs
table.  It is intended to eliminate the pathological case of
`FragmentedMapper` where a hash collision can result in many locking
operations to find the matching or vacant slab table entry.

FragmentedMapper was FragmentedMmapper in JikesRVM.  FragmentedMapper
may be a misnomer introduced when porting it to Rust.

The hash table design should have been a mistake.  Now that we are going
to replace it, we replace it completely.

x86_64 actualy has 47 bit address space for the user.

The current implememntation only has two levels.  We leave the name
"MultiLevelMmapper" to a true multi-level implementation in the future.