feat(len): add APIs to calculate component lengths

[zkat]

Fixes: #19

This adds some convenience methods that will make #19 possible, although it still needs some manual funging, unfortunately. There's no good way to maintain consistent spans across a document, partly due to limitations of how nom works, partly because, in practice, they would quickly go out of sync as soon as the KdlDocument or any of its components get edited.

So my suggestion is to use these new len() APIs to do manual offset calculations in your documents to come up with sensible spans and error messages for the particular failure cases of your KDL-based DSLs. You should then be able to throw the spans you come up with at miette to get some nice error message rendering, to boot! :)

Anyway, I hope this helps. It's unfortunate I couldn't come up with something that does more heavy lifting, and I'm always open to accepting patches to allow that, if anyone else ends up having any brilliant ideas on how to get that working right.