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/test/compile-fail/fn-constraint.rs

@@ -0,0 +1,9 @@
+// error-pattern: Unsatisfied precondition constraint (for example, le(a, b)
+use std;
+import std::str::*;
+
+fn main() {
+  let uint a = 4u;
+  let uint b = 1u;
+  log_err (safe_slice("kitties", a, b));
+}

src/test/compile-fail/impure-pred.rs

@@ -1,5 +1,5 @@
 // -*- rust -*-
-// error-pattern: impure function
+// error-pattern: Pure function calls function not known to be pure
 
 fn g() -> () {}
 

src/test/compile-fail/not-pred-args.rs

@@ -2,7 +2,7 @@
 
 // error-pattern: Constraint args must be
 
-fn f(int q) -> bool { ret true; }
+pred f(int q) -> bool { ret true; }
 
 fn main() {
 // should fail to typecheck, as pred args must be slot variables or literals