Improve coalescing register allocator (both verified and unverified versions)

[tanyongkiam]

The coalescing register allocator is one of the slowest parts of the compiler (~30% time is spent in it when running the compiler on its own s-expression).

This issue is about improving its performance, possibly with a rewrite/overhaul.

The external SML-based reg_alloc,sml oracle for in-logic compilation could be improved as well.