Get rid of tcg_fld_inst_env, replace it with tcg_axioms and correct use of tcg_type_env
Move FastStringEnv into a separate module
Refactor FieldLabels and their use in TyCons

Now an AlgTyCon carries a map from label strings to FieldLabels, so we
can efficiently decide whether it has a particular field and find
the corresponding selector Name.
Refactor lookup of records class instances to simplify
Don't need to make FamInsts in makeOverloadedRecFldInsts
Remove unwanted Eq and Ord instances for FieldLabel
ChildNames for overloaded record fields can refer to multiple selectors
Refactor axiom/dfun lookup and move to RnEnv