添加target：wasm32-unknown-unknown
info: downloading component 'rust-std' for 'wasm32-unknown-unknown'
安装wasm-pack
cargo install wasm-pack
Cargo安装可能会报openssl编译错误，建议直接下载：
https://github.com/rustwasm/wasm-pack
https://rustwasm.github.io/wasm-pack/