`$` doesn't work for CritBitTree[void]

[kaushalmodi]

Example code:

import critbits
var s: CritBitTree[void]
incl(s, "a")
echo s

gives this error:

nim_src_N4VsP0.nim(8, 6) template/generic instantiation from here 
lib/pure/collections/critbits.nim(325, 26) template/generic instantiation from here 
lib/pure/collections/critbits.nim(244, 43) Error: undeclared field: 'val'

---

This fails too:

import critbits
var s: CritBitTree[void]
echo s

[kaushalmodi]

Looks like echoing void critbittrees was intended:

proc `$`*[T](c: CritBitTree[T]): string =
  ## turns `c` into a string representation. Example outputs:
  ## ``{keyA: value, keyB: value}``, ``{:}``
  ## If `T` is void the outputs look like:
  ## ``{keyA, keyB}``, ``{}``.

but it's not working because of absent val.