Have a checksum performed on pre-built binaries being downloaded

First of all thanks a lot for the work on this tool, much appreciated.

Feature description

I was going to bump wasm-opt in wasm-pack due to the much better runtimes of newer versions and I saw it has been done already (thanks for that !).

I was wondering while planning the above if the prebuilt binaries should be submitted to a sha256 checksum to make sure there is nothing fishy going on.

Essentially the sha256 digests from github would be added as hardcoded strings in the code and using a sha256 crate, or a shasum executable, the integrity of the downloaded file could be checked.

I've taken a habit to run sha256 on downloaded artifacts in projects I work on and I didn't think about how wasm-pack was managing binaries until I had to dig into a fairly slow wasm-opt call.

Cheers