Modular Type checking for Functions

[DavePearce]

Original corset accepts purefun_09, whilst this currently reports an error. The issue is really arising from the fact that we are type checking within the body of the defun. This makes sense, but its not backwards compatible. Specifically, we have something like this:

(defun (id (x :binary)) x)
(defun (f x) (id x))

Currently, f will not compile because id requires its parameter to be :binary. The fix is very simple: explicitly declare f as accepting (x :binary).

[DavePearce]

Overall, I think this is actually something we shouldn't fix. The error message simply forces us to put a parameter on the enclosing defun declaration, which is entirely sensible. It means we'll have to update the constraints to match this. If there are a large number of problem sites, then we may need to revisit this decision.