Possible performance antipattern in formmanipulation

[wence-]

Describe the current issue

I happened to be looking at this code I wrote years ago and noticed:

firedrake/firedrake/formmanipulation.py

Line 72 in c9af09f

return self.expr(o, *map_expr_dags(self.index_inliner, operands))

This is the traversal antipattern discussed in FEniCS/ufl#35

In lieu of a proper fix in UFL to the visitor infrastructure, I think this could be addressed by passing in a cache as the optional argument to map_expr_dags (as seen in FEniCS/ufl#69)

[wence-]

I think the IndexInliner object could be removed and the handler for Indexed promoted to the ExtractSubBlock multifunction.