Wasm/web proof of concept

[sneurlax]

We can't use the exact same monero package on the web as we can on desktop and mobile platforms: instead of ffi, we must use eg. wasm_ffi. This will likely require changes to and possibly an alternative to native_toolchain_rust--we may have to build Rust manually, for example (and that might be a good shortcut to a quick proof of concept). I think if the tasks below are feasible that they'll be a quick path towards targetting web/WASM:

• Target wasm32-unknown-emscripten in monero-web crate (monero-web-rust)
• Wasm build
  either:
  • Automatically using native_toolchain_rust for web (WASM) builds. See Wasm/web targets irondash/native_toolchain_rust#23 .
    or:
  • Manually
• Replace ffi use with wasm_ffi.

[sneurlax]

See also https://github.com/cloudpeers/ffi-gen/