Enforce legality of variable names at (define)

[kantai]

Right now, almost all variables are fair game in (define) and (let), though the lookup path for reserved names prevents overriding +, etc.

A related issue is that in the interpreter, variables and functions are logically in two different namespaces (this helps with typing at the interpreter level, because functions are not value types). Of course, we want to prevent overlapping defines of variables/functions as well.

[kantai]

This was merged into #921