Add export! macros

[lann]

Also update bindings.rs with wit-bindgen 0.19.2.

Closes #76

use wasi::http::types::{IncomingRequest, ResponseOutparam};

wasi::http::incoming_handler::export!(Handler);

struct Handler;

impl exports::wasi::http::incoming_handler::Guest for Handler {
    fn handle(request: IncomingRequest, response_out: ResponseOutparam) {
        ...
    }
}

[alexcrichton]

This all looks great to me, thanks for both the suggestion and the implementation!

[alexcrichton]

Oh I realize now though that this isn't tested on CI. Can you add a CI check to ensure that the crate builds with this feature enabled?

For other nice-to-haves, feel free to defer these to either a future PR or a future contributor:

• These would probably make good entries in documentation at the crate level along with instructions of how to build.
• The wasi:cli/run macro may want some caution about it to indicate that it doesn't work with fn main() {} and fn main() {} otherwise works with the "command" adapter
• It'd be good to have a CLI check to ensure that these build the expected world, perhaps using wasm-tools component targets

Again though these points aren't critical, only the CI bit I think is critical to have before landing.

[lann]

Can you add a CI check to ensure that the crate builds with this feature enabled?
It'd be good to have a CLI check to ensure that these build the expected world, perhaps using wasm-tools component targets

Added wasmtime run for the cli/command example and wasm-tools component targets for the http/proxy example.

These would probably make good entries in documentation at the crate level along with instructions of how to build.

Added "Export Macros" section to root docs.

The wasi:cli/run macro may want some caution about it to indicate that it doesn't work with fn main() {} and fn main() {} otherwise works with the "command" adapter

Added "Compatibility with wasm32-wasi targets" section to macro docs.

[alexcrichton]

Thanks again for all this!