Add historical hydrate

[jeffa5]

I'm currently wanting to use this for restoring values in a document both at the latest state as well as sometimes from historical versions, using _at queries with the list of heads.

I can't see how to use autosurgeon's hydrate for this but I may be missing something. If it is missing I'd be happy to have a go at implementing it.

[alexjg]

One possibility would be to use fork_at to obtain a copy of the document as at the given heads and then hydrate from that.

@orionz is working on a reformulation of the ReadDoc trait which rather than having _at methods on it allows you to just obtain a ReadDoc as at a given set of heads. Once that lands we should be able to update autosurgeon to use that approach so you would be able to do something like:

let mydata: MyData = hydrate(doc.at(&heads))?

[jeffa5]

Ok, that sounds good. In the meantime I think I should be able to add a wrapper struct myself to use the _at calls when needed.

I think fork_at would be too expensive to be practical for this application but worth noting.