Add a combinator similar to `labelled`, but accepting a lambda that generates a label

[Hedgehogo]

This will avoid requiring the implementation of L to implement Clone in terms of combinators. It will simplify the implementation of whitespace without breaking the API and other text combinators

It might be worth making just a labelled_with combinator that accepts extra.

[zesterer]

I am in favour of this, and in fact I have needed this already for some internal features (notably, in #895).

labelled_with seems like a reasonable name.

It is a little bit of a shame that both can't easily be variants of the same combinator (at least, without some type shenanigans that are not worth implementing), but ah well.

Are you planning to work on this?

[Hedgehogo]

labelled_with seems like a reasonable name.

With the extra receiving lambda?

Yes, I think I will do that.

[Hedgehogo]

Added in ab7202a