Make ILinks more generic (support string and other types)

[konard]

The link address can be represented using any type.

There should be IUnsignedNumber constraint removed.

To do it, we might also introduce ILinksConstants, to be able to abstract away constants to be anything (like string and so on).

It is needed at https://github.com/link-foundation/link-cli.

We also might add IUnsignedLinks (as inherited from ILinks and IUnsignedNumber) with IUnsignedNumber constraint for backward compatibility.