Random: proc rand(x: HSlice) requires substraction

[mratsim]

Currently it is not possible to use rand with slices of char or enum:

import random

let slice = '0'..'F'
let r = rand(cslice)

Error: type mismatch: got <char, char>
but expected one of:
proc `-`[T](x, y: set[T]): set[T]
...

type
  Op = enum
    Halt
    Inc
    Dec

import random

let opslice = Halt..Dec
let op = rand(opslice)

Same error

[Araq]

But if we support enum the code could produce invalid values for enums with holes.

[mratsim]

Then I think the signature should be T: SomeInteger.

[Araq]

Fine with me.