Support undefined link syntax

[Konard]

:

and

(:)

and

()

Should be supported.

All above expressions should be equivalent to either:

(1: 1 1)

or

(1: 0 0)

or

(0: 0 0)

For Doublets, (should be configurable by LiNo interpreters, not the parser).

[Konard]

It can be useful to build short sets or nested structures that just have the same root, no matter which root.

:
  1 2
  3 4

May be transformed to:

x: x x
x (1 2)
x (3 4)

This syntax conflicts with: https://github.com/linksplatform/Protocols.Lino/issues/21