* Make more code work with no_std. no_std support is still incomplete, but this patch takes care of the bulk of the straightforward parts.
This adds a `--always-lightbeam` option as well as an `--always-cranelift` option, to allow the compilation strategy to be selected via the command-line. This also enables regular testing for Lightbeam.
* Migrate wasm-rust-api code to wasmtime.