PMA: Garbage collection

[eamsden]

The PMA needs a garbage collector. We need the following functionality:

• Deallocating unused blocks
• Melding small [what is our predicate for small?] allocations into larger allocations
• Tracing multiple roots
• Trait for tracing and melding
• [later extendable to] tracing multiple roots after checking that owning threads are still active (for concurrent scries)