Move unboxing code from Cmmgen to Flambda

[mshinwell]

In the future

[bluddy]

This would be really nice. I envision a cost model where if no generic operations are detected, the value can be safely unboxed, and the cost goes up relative to the number of generic operations along the way.

Ideally, of course, we could monomorphize most generic ops, making almost all floats/int64s unboxed all the time (with the obvious exception of functions stored inside data structures).