Add support for getting snapshot size

It'd be great if we could easily make the resulting snapshot's size available. For the CLI tool, perhaps just logging it to stdout, but in the embedding API having an accessor for it would be great.

And I guess there are at least two different sizes to be had here: the size added to the `wasm` file, and the size of the instance `Memory`. The latter might be more important, since the former is easy to derive.