Square brackets for generics

[Yardanico]

Solitude is right, but there is another problem with square brackets together 
with UFCS in Nim - a.b[T](https://forum.nim-lang.org/postActivity.xml) doesn't 
work, for that a new syntax extension was introduced - 
a.b[:T](https://forum.nim-lang.org/postActivity.xml) so it's not ambiguous.