Figure out correct rustc flags for rust >= 1.82.0

[zshipko]

It seems like after 1.81 rust requires some (nightly only?) flags to build the wasm code correctly. See https://blog.rust-lang.org/2024/09/24/webassembly-targets-change-in-default-target-features.html

[zshipko]

The only solution that works (which is from that link) requires switching to nightly. We need to figure out what the trade-offs are between switching to nightly or only supporting <= 1.81

[fliepeltje]

Regardless of how you end up solving this, it might be good to include a rust-toolchain.toml and amend it as you go. I've made a PR for this.