Make resolve recognize upvars

Upvars are now marked with def_upvar throughout, not just when going
through freevars::lookup_def. This makes things less error-prone. One
thing to watch out for is that def_upvar is used in `for each` bodies
too, when they refer to a local outside the body.

src/comp/syntax/parse/parser.rs

@@ -173,7 +173,6 @@ fn bad_expr_word_table() -> hashmap<istr, ()> {
     words.insert(~"prove", ());
     words.insert(~"native", ());
     words.insert(~"fn", ());
-    words.insert(~"block", ());
     words.insert(~"lambda", ());
     words.insert(~"pure", ());
     words.insert(~"iter", ());