Help wanted: Benchmarks in xray_core

It would be great to get a PR setting up benchmarks on xray_core in the canonical Rust manner. You can use your imagination on the kinds of operations we want to test, but here are some ideas:

• Making large numbers of edits
• Moving large numbers of cursors in a document containing large numbers of edits

Just getting a basic framework in place with some non-trivial benchmark would be helpful.