MPL-specific library?

[shwestrick]

Something I've been considering is making a MPL-specific structure with, for example:

• MPL.Parallel with fork/join primitives
• MPL.Concurrency with atomics, synchronization prims, etc.
• MPL.Unsafe with e.g. array allocation
• MPL.GC for GC statistics and other controls

This would help address the issues #106 and #77.

One big advantage of doing it this way is that we can stay "compatible" with MLton, in the sense that we can sync the MLton library with mainline MLton. In MPL, we would just explicitly deprecate certain functionality (e.g. MLton.GC.pack would raise a "not supported" exception)