Re-introduce optional dedicated scratch registers (#117)

* Re-introduce optional dedicated scratch registers

Dedicated scratch registers used for resolving move cycles were removed
in #51 and replaced with an algorithm to automatically allocate a
scratch register as needed.

However in many cases, a client will already have a non-allocatable
scratch register available for things like extended jumps (see #91). It
makes sense to re-use this register for regalloc than potentially
spilling an existing register.

* Clarify comment

src/ion/moves.rs

@@ -986,6 +986,9 @@ impl<'a, F: Function> Env<'a, F> {
                     to: pos_prio.pos.next(),
                 });
                 let get_reg = || {
+                    if let Some(reg) = self.env.scratch_by_class[regclass as usize] {
+                        return Some(Allocation::reg(reg));
+                    }
                     while let Some(preg) = scratch_iter.next() {
                         if !self.pregs[preg.index()]
                             .allocations