Add Counted mixin class and refactor form signature computation

[connorjward]

In Firedrake we have been running into form signature stability problems with our Constants since they are implemented differently to how they are done in UFL. Specifically we need to add them to renumbering when the form signature is calculated.

I have refactored the counted_init part of UFL and replaced it with a Counted mixin class. This means that we can compute the renumbering for all instances of Counted, rather than specifically for Coefficients, Constants etc.

[connorjward]

@mscroggs please could you take a look at this? Tests appear to be failing because of some unrelated linting issues. I suspect flake8 have released a new (stricter) version.