Add ability to customize bindings output path without BC breaks.

[gcoguiec]

Hi!

Thank you for the fantastic work on cargo-component!

This is PR is an alternative proposal to #316 that aims to address #315 in a backward-compatible manner.

What could be added to this PR furthermore?

• Test cases
• Documentation update

How to change bindings output path using code in this PR

1. Move the component declaration to a dedicated module of your choosing (here wasi):

// src/wasi.rs
pub mod bindings;

use bindings::Guest;

struct Component;

impl Guest for Component {
  ...
}

bindings::export!(Component with_types_in bindings);

(component declaration stays the same)

2. Register the new wasi module inside your lib:

// src/lib.rs
#[cfg(target_os = "wasi")] // ymmv
pub mod wasi;

3. Change bindings generation path to src/wasi/bindings.rs by adding a metadata inside the Cargo.toml manifest:

# Cargo.toml
[package.metadata.component.bindings]
path = "src/wasi/bindings.rs"

This way, we don't break current behaviour while allowing the bindings output path to change.