Continued sketching out code for checking states against preconditions.

It's still sketchy. I added a typestate annotation field to statements
tagged stmt_decl or stmt_expr, because a stmt_decl statement has a typestate
that's different from that of its child node. This necessitated trivial
changes to a bunch of other files all over to the compiler. I also added a
few small standard library functions, some of which I didn't actually end
up using but which I thought might be useful anyway.

src/lib/util.rs

@@ -11,6 +11,18 @@ fn rational_leq(&rational x, &rational y) -> bool {
     ret x.num * y.den <= y.num * x.den;
 }
 
+fn fst[T, U](&tup(T, U) x) -> T {
+    ret x._0;
+}
+
+fn snd[T, U](&tup(T, U) x) -> U {
+    ret x._1;
+}
+
+fn orb(&bool a, &bool b) -> bool {
+    ret a || b;
+}
+
 // Local Variables:
 // mode: rust;
 // fill-column: 78;