Bind and switch (and other improvements)

[countvajhula]

Summary of Changes

This is our WIP from the most recent Qi meeting, including:

• a binding specification for if (and therefore switch) that allows conditions to bind clauses (see: the ol' bind and switch)
• remove fixed-point finding in deforestation pass as a single pass is sufficient
• more unit tests for bindings
• starter tests for the expander
• use the de-expander to improve more error messages

Public Domain Dedication

• In contributing, I relinquish any copyright claims on my contribution and freely release it into the public domain in the simple hope that it will provide value.

[countvajhula]

FYI @michaelballantyne @dzoep I'm merging these changes which are mostly what we talked about yesterday. One concern is that the "de-expander" is basically matching (but not validating) the whole core language in order to address #134 . But in flow.rkt where we need this, it feels like we could already have the unparsed surface syntax, but I'm not sure if there is a way to just keep it around in addition to passing it to closed-floe expander. Of course, when "get-surface-syntax" is available from syntax-spec that would also solve this.

P.S. No action needed, enjoy your vacation!