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/fuzzing/func.rs

@@ -662,11 +662,13 @@ pub fn machine_env() -> MachineEnv {
     }
     let preferred_regs_by_class: [Vec<PReg>; 2] = [regs(0..24), vec![]];
     let non_preferred_regs_by_class: [Vec<PReg>; 2] = [regs(24..32), vec![]];
+    let scratch_by_class: [Option<PReg>; 2] = [None, None];
     let fixed_stack_slots = regs(32..63);
     // Register 63 is reserved for use as a fixed non-allocatable register.
     MachineEnv {
         preferred_regs_by_class,
         non_preferred_regs_by_class,
+        scratch_by_class,
         fixed_stack_slots,
     }
 }