Implement more type environments

[bennn]

Turnstile re-uses the macro expander to implement good old lexical scope. That's great.

Turnstile has the full power of Racket, so programmers can implement their own scoping rules. That's great. I think ...

... can turnstile implement more "Gammas", and let the user choose between some common ones as another kind of hook?

[Stephen Chang]

The #:modes parameter added by @iitalics attempts to allow this. See the linear lang for an example usage. https://bitbucket.org/stchang/macrotypes/src/3a02510a0f038c4eb342faa279ff51347f9fd41b/turnstile/examples/linear-var-assign.rkt?at=def-typed-var-stx&fileviewer=file-view-default

[Milo Turner]

https://github.com/iitalics/macrotypes/blob/master/turnstile/examples/linear/lin.rkt is better organized imo

[Stephen Chang]

Great, thanks.