Convert regalloc::coloring to use an EncCursor.
Convert regalloc::coloring to use an EncCursor.

No functional change intended, this is just a big fight with the borrow
checker.

stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,sunfishcode/cretonne,stoklund/cretonne