Disentangle stxutils from Cur

[wilbowma]

Commit 659c8d3 adds some stxutils, (part of #79) which seemed like they would be generally useful. Unfortunately, I can't disentangle them from Cur (all stxutils ought not to depend on Cur). The new subst and stx=? functions apparently needs to know about binding structures, and find-in apparently needs to be able to do identifier equality by name instead of by scopes. The new ntac tactics rely on breaking hygiene abstractions in these ways due to some issue with scopes.

I've tried abstracting them a bit here: https://gist.github.com/wilbowma/477954b228e94bd682bffdc4d15ff0bf
but I can't get the new versions to work, and I'm not even sure the approach makes sense.

@stchang, if you have thoughts, otherwise I think I'll just move the utils into rewrite.rkt until we sort out the scope issues.

[stchang]

One possibility is putting them in a separate collect?

Otherwise, I'm fine with putting them in rewrite.rkt or having a separate stx-utils.rkt in cur/ntac.