Consider adding an API that takes the nonce size and tag size as parameters

We are building a Node.js runtime for the browser so you can run Node.js entirely in your browser without relying on cloud servers. That means we run arbitrary code and right now it's only possible to specify the nonce size and tag size statically via types so we are forced to always call from_slice(...). Ideally we can create it once and not rely on a match that matches nonce size and tag size, e.g.

 match nonce.len() {
   12 => match tag_size {
     12 => AesGcm::<Aes, U12, U12>::new_from_slice(key),
     ...
   },
   ...
 }

So it'd be nice if we could do from_slice(key, nonce_size, tag_size).

Start by locating the AesGcm API and its current from_slice(...) constructor, then review how nonce and tag sizes are represented in the type parameters. Determine the intended behavior for runtime nonce and tag sizes; done means the API design is agreed and its supported size combinations are covered by tests.