Implement pattern guards

The syntax is

    alt x {
        mypat where mycond { ... }
    }

The condition may refer to any of the variables bound by the pattern.
When a guard fails, pattern-matching continues with the next pattern.

Closes #857

src/comp/syntax/parse/parser.rs

@@ -1360,8 +1360,12 @@ fn parse_alt_expr(p: &parser) -> @ast::expr {
     let arms: [ast::arm] = [];
     while p.peek() != token::RBRACE {
         let pats = parse_pats(p);
+        let guard = none;
+        if eat_word(p, "when") {
+            guard = some(parse_expr(p));
+        }
         let blk = parse_block(p);
-        arms += [{pats: pats, body: blk}];
+        arms += [{pats: pats, guard: guard, body: blk}];
     }
     let hi = p.get_hi_pos();
     p.bump();