`let` errors are misleading on mismatched destructuring arguments

[stanislowskij]

Originally a case in #5.

babel.middleware=> (let [[b c] 4 a (inc 2)] (+ a b c))

Function nth does not allow a number as an argument.

let destructuring uses nth to try pairing the de-structured vector [b c] with another vector, but is given 4 instead. It cannot take the nth argument of 4.

This may be part of a more general problem with handling de-structuring.