Add new functions to generate rust bindings in build.rs

[tomasol]

Add RustWasm::generate_to_out_dir that parses the wit folder, selects a world, generates bindings and places them in a file in the OUT_DIR. generate_to_out_dir_modify allows modifying the generated file.

Sample build.rs

use anyhow::Result;
use std::borrow::Cow;
use wit_bindgen_rust::Opts;

fn main() -> Result<()> {
    Opts {
        generate_all: true,
        additional_derive_attributes: vec![
            "serde::Serialize".to_string(),
            "serde::Deserialize".to_string(),
        ],
        ..Default::default()
    }
    .build()
    .generate_to_out_dir_modify(Some("any"), |contents| {
        let contents = String::from_utf8(contents.to_vec()).unwrap();
        let re = regex::Regex::new(r"(pub\s+enum\s+\w+)").unwrap();
        Cow::Owned(
            re.replace_all(&contents, "#[serde(rename_all = \"kebab-case\")]\n$1")
                .into_owned()
                .into_bytes(),
        )
    })
}

Then the following snipped would include it:

mod generated {
    include!(concat!(env!("OUT_DIR"), "/wit_bindgen_generated.rs"));
}

Although this should be simplified as well.

Closes #1423

[alexcrichton]

Instead of using a hardcoded name could all the files be written with the name from the generator itself?

[alexcrichton]

Mind adding some doc comments for these methods?

[alexcrichton]

This isn't necessarily tightly integrated into the generation step, so could this be omitted and performed as part of the top-level build itself? For example the caller of generate_to_out_dir would read the output file, perform the transformation, and then write back out the contents.