Efficiency: don't recalculate len() and weight()

Currently, the weight and length of each node is calculated again and again. This is, so that I can concentrate on the API, reference counting, `Rope`s operations, etc. Eventually, len() and weight() will both be removed with `Lazy` or something like that...

Start by locating the Rust implementations and call sites for len() and weight() across the rope nodes. Read how the API, reference counting, and Rope operations depend on these calculations, then clarify the intended caching approach; done means repeated length and weight calculations are avoided without breaking those areas.

Written by the indexing model from the issue text.