add $stx as a binding instance for parsing syntax-objects like syntax-parse and allow just an id in define-match-bind

```racket
> (~define ($stx (a b c)) #'(1 2 3))
> #'a
#<syntax 1>
> (~define ($stx ((~seq kw:keyword arg:expr) ...)) #'(#:a 1 #:b 2 #:c 3))
> #'(kw ...)
#<syntax (#:a #:b #:c)>
> (define-match-bind hash-table)
> (~define ($hash-table ['a a] ['b b] ['c c]) (hash 'a 1 'b 2 'c 3))
> a
1
> b
2
> c
3
```