Restructure the "checking" pass in typestate

I noticed that typestate was being lazier than it should be,
because it was only checking typestate for statements and
top-level expression (that is, the expression in a stmt_expr, but
not any subexpressions). So I rewrote the checks in tstate/ck.rs
to use walk, which exposed a few bugs in typestate that I fixed.

Also added some more test cases for if-check.

src/test/compile-fail/if-check-precond-fail.rs

@@ -0,0 +1,30 @@
+// xfail-stage0
+// error-pattern:Unsatisfied precondition constraint
+pred even(uint x) -> bool {
+  if (x < 2u) {
+    ret false;
+  }
+  else if (x == 2u) {
+    ret true;
+  }
+  else {
+    ret even(x - 2u);
+  }
+}
+
+fn print_even(uint x) : even(x) {
+  log x;
+}
+
+fn foo(uint x) -> () {
+  if check(even(x)) { 
+      fail;
+    }
+  else {
+    print_even(x);
+  }
+}
+
+fn main() {
+  foo(3u);
+}