Implement bitsets

[Araq]

Size computation already available in evalexprs.nim.

One way to implement these rather easily:

func `[]`[T](a: set[T]; i: int): var uint8 {.magic: "AccessSetAsByteArray".}

func `+`*[T](a, b: set[T]): set[T] {.inline, noinit.} =
  for i in 0 ..< sizeof(a): result[i] = a[i] or b[i]

func `*`*[T](a, b: set[T]): set[T] {.inline, noinit.} =
  for i in 0 ..< sizeof(a): result[i] = a[i] and b[i]

func `-`*[T](a, b: set[T]): set[T] {.inline, noinit.} =
  for i in 0 ..< sizeof(a): result[i] = a[i] and not b[i]

func `==`*[T](a, b: set[T]): bool {.inline.} =
  for i in 0 ..< sizeof(a):
    if a[i] != b[i]: return false
  return true

func `<=`*[T](a, b: set[T]): bool {.inline.} =
  for i in 0 ..< sizeof(a):
    if (a[i] and not b[i]) != 0: return false
  return true

func `<`*[T](a, b: set[T]): bool = (a <= b) and not (a == b)