* 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
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