add $stx and allow just an id in define-match-bind
Merge pull request #8 from AlexKnauth/$stx

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

Is there anything I’m missing or should add or anything?
And what is the purpose of the nested-definers and the nested-idss?
And should I do something for let-only?
And why does ~let use definer instead of letter?  Wouldn’t that make it
behave like letrec?