Add flag to enable AST rewriting

Problem

Since we have both an offline (check-ns) way to type check, and online
(typed REPL), it only makes sense to rewrite the AST in certain cases.

Solution

Add a dynamic variable can-rewrite which is true when AST rewriting
makes sense.

Pull request: 41
Patch: ctyp-254.patch
Commit: 9e7b73
Release: 0.3.9