Add kind-checking for assign-op, copy, ret, be, fail exprs. Fix caught kinding-violations in rustc and libstd.

src/comp/syntax/ast_util.rs

@@ -3,7 +3,7 @@ import std::option;
 import codemap::span;
 import ast::*;
 
-fn respan<T>(sp: &span, t: &T) -> spanned<T> { ret {node: t, span: sp}; }
+fn respan<@T>(sp: &span, t: &T) -> spanned<T> { ret {node: t, span: sp}; }
 
 /* assuming that we're not in macro expansion */
 fn mk_sp(lo: uint, hi: uint) -> span {