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Make for _ in _ loops use pattern matches.

    Now, the syntax for 'for x in y' will use a pattern match
    for x, and only matching elements will be iterated over.

--- a/parse/gram.y

+++ b/parse/gram.y

@@ -690,7 +690,7 @@

 forstmt : Tfor optexprln optexprln optexprln block

             {$$ = mkloopstmt($1->line, $2, $3, $4, $5);}

-        | Tfor expr Tin exprln block 

+        | Tfor pat Tin exprln block 

             {$$ = mkiterstmt($1->line, $2, $4, $5);}

         /* FIXME: allow decls in for loops

         | Tfor decl Tendln optexprln optexprln block

--- a/parse/infer.c

+++ b/parse/infer.c

@@ -1318,10 +1318,17 @@

             constrain(st, n, type(st, n->loopstmt.cond), cstrtab[Tctest]);

             break;

         case Niterstmt:

-            infernode(st, n->iterstmt.elt, NULL, sawret);

+            bound = NULL;

+            nbound = 0;

+

+            inferpat(st, n->iterstmt.elt, NULL, &bound, &nbound);

+            addbindings(st, n->iterstmt.body, bound, nbound);

+

             infernode(st, n->iterstmt.seq, NULL, sawret);

-            infernode(st, n->loopstmt.body, ret, sawret);

-            constrain(st, n, type(st, n->loopstmt.cond), cstrtab[Tctest]);

+            infernode(st, n->iterstmt.body, ret, sawret);

+

+            constrain(st, n, type(st, n->iterstmt.seq), cstrtab[Tcidx]);

+            unify(st, n, type(st, n->iterstmt.elt), type(st, n->iterstmt.seq)->sub[0]);

             break;

         case Nmatchstmt:

             infernode(st, n->matchstmt.val, NULL, sawret);