- Optimize zonking * to avoid allocation.
- Try to avoid looking at the free variables of a type in the
  pure unifier. We need look at the variables only in the case
  of a forall.

The performance results updates included in this also include
a regression, but the regression is not due to this patch. When
validating previous patches, the test case failed, but I was
unable to reproduce outside of validation, so I let it go by, thinking
the failure was spurious. Upon closer inspection, the perf number
was right at the line, and the wibble between a valiation environment
and a regular test run was enough to make the difference.