cross platform solution for rand at CT (avoidining 'dummy seed' of windows and other cross platform issues)

[timotheecour]

/cc @ba0f3

potential cross platform solution for rand at CT (avoidining 'dummy seed' of windows and other cross platform issues):

• make nimble install tempfile installs a binary (these go automatically in ~/.nimble/bin IIRC, on posix), say, tempfile_helper
  this binary returns the seed random.randomize() (which we can improve later, independently), or a custom proc, and prints its value
• call it at CT using gorgeEx (let's say we can assume it's in PATH, or maybe allow passing a string in API to specify path to bin containing it)

note

• this would fix date +'%N' doesn't work on OSX #11
• this would fix windows case at CT (currently not really working because of dummy seed)
• this would be more robust than relying on date +'%N' , and allow making it further more random by allowing to use pid and threadid, all in a clean way using pure nim code

[ba0f3]

too bad, getpid proc and times module does not work on compile time.

[ba0f3]

problem will be resolved once this PR get merged nim-lang/Nim#9851, refs nim-lang/Nim#8588

here is an example for getting random seed at compile time

import random

proc seeder() = discard
proc getSeed(p: pointer): int =
  cast[int](p)

proc test(): int {.compileTime.} =
  var
    seed = getSeed(seeder)
    r = initRand(seed)

  for i in 1..10:
    echo r.rand(int.high)

discard test()