In this example we don't need the request buf.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

There is no need in this case to declare REQUEST_BUF as a global
variable. Declaring it local to uwr_request_handler() lets us get rid of
the unsafe code blocks.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

This is about the smallest it can be.

Its Unit application config would look like

    "applications": {
        "rust-hello-world": {
            "type": "wasm",
            "module": "/path/to/unit-wasm/examples/rust/hello-world/target/wasm32-wasi/debug/rust_hello_world.wasm",
            "request_handler": "uwr_request_handler",
            "malloc_handler": "luw_malloc_handler",
            "free_handler": "luw_free_handler"
        }
    }

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>