Support implicit returns

[arnetheduck]

• based on https://github.com/nim-lang/Nim/pull/21898
• also fixes generic Future[T] where T ends up being void

supercedes #349 & #328

[cheatfate]

I think test suite is not complete, i wish to see case, block, try/except/finally in tests. Also it would be interesting how it works with things like this:

return
  case value
  of A:
    1
  of B:
    2
  of C:
    return 2023

[arnetheduck]

i wish to see

ffd6005d085acc3d8ffaf9cc76d5c7fb227e460a

Also it would be interesting

added a variation without one of the returns