Optimized WebAssembly "touching" routines

[adroitwhiz]

Proposed Changes

This is a project I worked on for fun last April, which moves the software implementations of "touching" blocks from JavaScript to a WebAssembly module coded in Rust.

Reason for Changes

I benchmarked this on this project and on Chrome, it triples the speed of the "touching sprite" block.

This is a draft because the tooling is a bit rough around the edges and could stand to be better-documented, but I want to gauge interest in this.

You can test it out for yourself at https://adroitwhiz.github.io/scratch-gui/swrender/ though it's quite an old build of Scratch at the moment.

Test Coverage

All existing tests (should) pass

[GarboMuffin]

I need to spend more time reviewing this and making sure it doesn't break things, but this is very interesting and a more mature version would probably be merged.

[GarboMuffin]

I did some testing. Compatibility and performance are overall very good. Current blockers:

• Pen layer collision is broken, seems to be caused by removal of call to skin.updateSilhouette in f995aa3#diff-974fae2981f18c8c4be7493c4cc1c9ed55b5c0f8977b34ac301cf699e4eeff7bL656-L657
• It seems the wasm-bindgen CLI has been deprecated in favor of wasm-pack, is it worth switching?
• What does this mean for the GPU-based touching? Right now it seems that CPU-based touching is forced, but I assume that's temporary?

I can deal with the merge conflicts.