it back to a string (will raise an error if it's actually non-ASCII).

Conflicts:
	pymbolic/interop/sympy.py

Flake8 during ci

See merge request !2

Run Flake8 during CI

See merge request !1

More unifier fixes.

unify().

Fixes to unification records:
  - Properly merge rmap.
  - Don't store redundant equations.

Other fixes:
  - Fix an issue where existing urecs weren't properly being merged
    in map_commut_assoc().

Fix avoidable exponential blowup in unifier.

Change the code to do what the previous code tried to do, which is to
match free variables only after matching the more complex terms. This
avoids iterating over all possible subsets of the RHS.