Draft: Type inference for macros

[sheaf]

This PR implements a simple type-inference pass for macros,
see HsBindgen.C.Tc.Macro. It implements a type system with:

• variables
• function types
• type constructors
• quantified types

Type inference for a macro function definition proceeds in the
following stages:

1. constraint generation following Algorithm W, creating new
   metavariables and carrying a substitution,
2. constraint solving: constraint simplification and defaulting
3. quantification over remaining (undefaulted) type variables
   and remaining (unsolved) constraints.