Rename everything

I'm releasing this now, before waiting for CI, to avoid risk of crate name gazumping/front-running.

Unusually, not every commit in this MR will build and work. But the commits come in groups with a common prefix, and everything is good after every commit.

There are still some internal renamings and docs tidyings and so on left to do. This branch is the minimum for a coherent set of renames.