Closes #692

These changes only affect `PatternSub`, which now no longer allows constraints
on non (logic) variable terms in its patterns.  Likewise, the `values_eq_approx`
and `skip_identities_fn` options are no longer supported.

This commit introduces a `LocalOptTracker` object that performs an MRO-based
lookup of `LocalOptimizer`s that track `Op` types.

This improvement is due to the use of `x[indices] += y` instead of `np.add.at(x,
indices, y)`, but comes at the cost of not handling duplicate `indices` values.

Closes #97

These changes remove the tests' reliance on string output and make the expected
results much more explicit.