Add none[T]() as alias to none(T)

[metagn]

It's a direct alias in case there are PRs that change none(T), and deprecating or removing none(T) would be a bit of an off move

Could be used for using none[T] as a direct proc value instead of proc: auto = none(T)

[GULPF]

Does those examples work? I don't think Nim has that kind of type inference. E.g this doesn't compile:

var s: seq[int] = newSeq()

[metagn]

Oh, you're right

[Araq]

Please add test cases.

[metagn]

Added some