Some refactoring

Small shortenings here and there
I personally would rather have the lambda here.  To me seeing (define-syntax (~define/contract stx) ...) makes it look at a glance like the form is meant to take one argument, whereas if I see (define-syntax ~define/contract (lambda (stx) ...)), it makes it clearer that ~define/contract itself is not a function, but a macro that uses a function to parse its syntax.  But that's just my preference.  It doesn't matter much.