date +'%N' doesn't work on OSX

[timotheecour]

• date +'%N' doesn't work on OSX:
  (was introduced here: 1e64c88)
  date +'%N'
  N

• there's perhaps gdate if user installed it via brew install binutils, eg:
  gdate +'%N'
  225945000

• maybe it'd be a good idea to mixin thread-id and/or pid to avoid potential clashes where 2 threads calling same code generate same seed (hence, same tempfile)

• in D, unpredictableSeed does a mix of pid, threadid, and time:
  https://github.com/dlang/phobos/blob/e87b111162df48db747d535715817a37cefba6b1/std/random.d#L1685

• this is very related to these issues:

  • [feature] random.unpredictableSeed, from pid, threadid, time · Issue #8588 · nim-lang/Nim
  • random doesn't generate uniform numbers (very high correlation) when using seed from time · Issue #8589 · nim-lang/Nim
  • genOid has low entropy (creates lots of 0 bits) · Issue #9741 · nim-lang/Nim

there really should be 1 function that could be called for all these cases (or possibly, 1 for CT case and 1 for RT case), but happy to have it working in isolation in tempfile.nim for now

note

see #12 for a robust way to solve this