Function definitions

- export proofs bout alpha_bn


Parser should check that:

- types of bindings match types of binding functions
- fsets are not bound in lst bindings
- bound arguments are not datatypes
- binder is referred to by name and not by type

Smaller things:

- maybe <type>_perm whould be called permute_<type>.simps;
  that would conform with the terminology in Nominal2


Other:

- nested recursion, like types "trm list" in a constructor

- store information about defined nominal datatypes, so that
  it can be used to define new types that depend on these