Further reduce the core language

[countvajhula]

Recently we did some work to identify a core language to which the standard Qi language can be reduced. Having a small core language makes it easier to implement general optimizations, but in addition, it can also make it easier to implement Qi for backends other than simple Racket functions.

Currently there are something like 30-35 core forms (and about the same number of non-core forms). It would be useful to reduce this further.