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/isa/registers.rs

@@ -147,8 +147,8 @@ impl RegInfo {
 
 /// Temporary object that holds enough information to print a register unit.
 pub struct DisplayRegUnit<'a> {
-    pub regunit: RegUnit,
-    pub reginfo: &'a RegInfo,
+    regunit: RegUnit,
+    reginfo: &'a RegInfo,
 }
 
 impl<'a> fmt::Display for DisplayRegUnit<'a> {