Desugar: adding FFI type signatures on toplevel

* Add a desugar step which copies the type signature for toplevel FFI
  declarations to an explicit one in the RHS expression.
* Remove (now unnecessary) code from compileDecls and compileDecl

Still todo:
* Adding FFI type signatures in let/where bindings
* Find out why StrictWrapper test is broken. Seems to be ignoring the
  last statement in the do block.