Well-formedness rules for fn-def types

[nikomatsakis]

We need tests that fn-def types are well-formed (and maybe code?). To be well-formed, the where-clauses on the fn must be met. (That said, in the MIR type checker, I am currently adding in implications that will basically always make this true.)

[shua]

I may have addressed this with #181 . At least, the type parameters are validated there, the function body is still not checked.