Only non-keyword symbols should be bindable

[porcuquine]

Lurk currently allows binding any form at all as the 'variable' on the left-hand-side of a binding in let or letrec.

However, only symbols are ever looked up when evaluating.

It should be an evaluation-time syntax error to perform these useless bindings. Allowing them is confusing, can mask actual errors, and also has a performance cost (in other lookups).

Some example of the behavior we should eliminate follow. All of these should be explicit errors.

Lurk REPL welcomes you.
> (let (((a b) 123)) 1)
[3 iterations] => 1
> (let ((:a 123)) :a)
[3 iterations] => :A
> (let ((9 123)) 9)
[3 iterations] => 9