Properly typecheck and compile invocations of generic methods.

Aligning the type parameters of the ifaces, impls, and methods
correctly in typeck is almost brain surgery. Seems to work now for
everything I threw at it, but might still break in other corner cases.

Issue #1227

src/libcore/vec.rs

@@ -742,6 +742,17 @@ fn iter<T>(v: [const T], f: block(T)) {
     iteri(v) { |_i, v| f(v) }
 }
 
+/*
+Function: iter2
+
+Iterates over two vectors in parallel
+
+*/
+fn iter2<U, T>(v: [U], v2: [T], f: block(U, T)) {
+    let i = 0;
+    for elt in v { f(elt, v2[i]); i += 1; }
+}
+
 /*
 Function: iteri