More CallArity to its own module
Do not say “TailCall” in function names
Add the usual flags -fcall-arity and -dddump-call-arity
Stop fixpointing when below exprArity
Add huge note to CallArity

(the module is now 50% notes).
Bugfix: Properly remove bound variables in returned CallArityEnv
Add a unit test for CallArity

I put it all in one file because starting the GHC API is quite slow.