Serialize constraints in types (literal arguments still not supported)

This involved, in part, changing the ast::def type so that a def_fn
has a "purity" field. This lets the typechecker determine whether
functions defined in other crates are pure.

It also required updating some error messages in tests. As a test
for cross-crate constrained functions, I added a safe_slice function
to std::str (slice(), with one of the asserts replaced with a
function precondition) and some test cases (various versions of
fn-constraint.rs) that call it. Also, I changed "fn" to "pred" for
some of the boolean functions in std::uint.

src/comp/middle/ty.rs

@@ -2610,7 +2610,7 @@ fn substitute_type_params(&ctxt cx, vec[ty::t] substs, t typ) -> t {
 
 fn def_has_ty_params(&ast::def def) -> bool {
     alt (def) {
-        case (ast::def_fn(_)) { ret true; }
+        case (ast::def_fn(_,_)) { ret true; }
         case (ast::def_obj(_)) { ret true; }
         case (ast::def_obj_field(_)) { ret false; }
         case (ast::def_mod(_)) { ret false; }