`#` can't be used on the rhs of `letstx`

Created by: natefaubion

macro test {
  case { _ } => {
    letstx $foo = #{ 1 + 2 + 3 };
    return #{
      $foo;
    }
  }
}
test

This is because # breaks hygiene, referring to name_stx and match, and letstx defers to helper macros, so it gets the wrong name_stx and match when calling transcibe. Also illustrated by http://bit.ly/1fmrUwf

To clarify, this is a problem with # (well, really all fundamental macros that break hygiene), I just titled the issue the way I did because it's how you'd likely happen upon it.