More procs for dealing with CStringArray

cstrings.nim

proc allocCStringArray*(a: openArray[string]): cstringArray =
  ## creates a NULL terminated cstringArray from `a`. The result has to
  ## be freed with `deallocCStringArray` after it's not needed anymore.
  result = cast[cstringArray](alloc0((a.len+1) * sizeof(cstring)))
  for i in 0 .. a.high:
    # XXX get rid of this string copy here:
    var x = a[i]
    result[i] = cast[cstring](alloc0(x.len+1))
    copyMem(result[i], addr(x[0]), x.len)

proc deallocCStringArray*(a: cstringArray) =
  ## frees a NULL terminated cstringArray.
  var i = 0
  while a[i] != nil:
    dealloc(a[i])
    inc(i)
  dealloc(a)