Weak links: add support for subdocuments and unbounded ranges

[Horusiath]

With weak links released as part of v0.17, this is a proposal for the future improvements.

First, in their current shape, Quotable::quote can only allow us to use bounded ranges on both ends. It would be nice if it could also work over unbounded ranges as well.

Second, one of the very useful attributes of weak links would be an ability to link data between document and subdocuments as well.

The major feature I'd love to see here is that subdocuments are not necessarily loaded at the beginning. They may be just small references waiting for being loaded when they will be necessary. This nicely corresponds with WeakRef::try_deref/WeakRef::unqote/WeakRef::get_string etc. methods as they are lazily evaluating linked value and give us explicit trigger that may be used to ie. tell the subdocument to load the data.