`let` statement should evaluate to void

[ckknight]

Currently, if let is used as an expression, e.g.

if true
  let x = 5

or

#
  let x = 5

It evaluates to the value of x, unless it's a dereference, at which point it uses the last value retrieved.

It would be better to simply have it evaluate to void explicitly, which would then allow for temporary variables to be null'd out.