Add buffer benchmarks

[lhecker]

Also includes:

• criterion update to v0.6
• Simplify range parameter for GapBuffer::extract_raw
• Allow deleting multiple units via TextBuffer::delete
• And some additional smaller improvements

[lhecker]

I'm not 100% sure if this is sufficiently attributed. It's how https://github.com/josephg/editing-traces itself quotes the license, however.

The added archive is quite large (relative to the size of the repository), but I figured it's way too small for LFS, and still worth having long term. It's one of the few pieces of "realistic" editing traces out there = worth benchmarking.