toolchain research and choice

[Willi Ballenthin]

this probably isn't going to be as simple as cargo build:

• rustwasm/team#291
• rustwasm/wasm-bindgen#2209
• https://www.reddit.com/r/rust/comments/i8snc5/compiling_rust_library_with_c_dependencies_to_wasm/
• rust-lang/rust#63626
• WebAssembly/tool-conventions#88