Add an AllocatableSet for registers.

This set of available register units also manages register aliasing in
an efficient way.

Detect if the units in a register straddles mask words. The algorithm
for allocating multi-unit registers expect the whole register to be
inside a single mask word. We could handle this if necessary, but so far
no ISAs need it.

lib/cretonne/src/regalloc/mod.rs

@@ -4,3 +4,4 @@
 
 pub mod liverange;
 pub mod liveness;
+pub mod allocatable_set;