Add __foreach_active statement to loop over active prog. instances.

For now this has the __ prefix, as an experimental feature currently only
used in the standard library implementation.  It's probably worth making
something along these lines an official feature, but I'm not sure if this
in its current form is quite the right thing.

stmt.h

@@ -459,4 +459,7 @@ public:
     Expr *expr;
 };
 
+extern Stmt *CreateForeachActiveStmt(Symbol *iterSym, Stmt *stmts, 
+                                     SourcePos pos);
+
 #endif // ISPC_STMT_H