Extract IdMap and publish as a standalone crate

[jgallagher]

... and now that I'm thinking about it, this seems like a really nice thing to have available more generally! I feel like this specific thing (wanting to store data by key, but also putting the key in the data) shows up repeatedly, and having a wrapper which enforces consistency seems extraordinarily useful! I know I'd have used it in a little hobby thing I was doing just a couple months ago.

Originally posted by @sunshowers in #7327 (comment)