breaking change: Remove worker::send entirely

[guybedford]

Mulling this one over as an alternative to #784.

This uses a new wasm bindgen feature for unsafe-single-threaded-traits which just makes everything Sync and Send for us under the single-threaded assumption.

Would simplify things not having to have this noise in Rust Workers codebase.

[guybedford]

Ahh, found the failures here - I see this is because of Axum.

[WERDXZ]

Cases like leptos ssr do want server functions to return a future that is Send, this (the send module) become useful when downstream users needs to make futures Send in workers environment, especially like the JsFuture type.

It might not be a great example of usage, but I am using it in my website.

[guybedford]

I've updated this PR to use an experimental wasm bindgen feature for this in wasm-bindgen/wasm-bindgen#4770, and this approach seems to work now.

[guybedford]

All tests are passing - CI failure here is due to the examples build not using the forked wasm bindgen worker-build.