change syntax-case to allow internal definitions

[spdegabrielle]

change syntax-case to allow internal definitions (possibly by getting rid of the 'guard' section)

from Wishlist of backwards incompatible things for a future Racket2. #33

[jackfirth]

What if we just got rid of syntax-case altogether?

[sorawee]

I think it still makes sense to have syntax-case for low-level core libraries, and it could even be exported in a way that discourages its use.

Another thing that people tend to forget when using syntax-case (and even syntax-parse!) is tracking disappeared-use. It might make sense to have #:track-literals on by default for syntax-parse, and it must be opt-out if users don't want it.