Loosely integrate implements into bindgen!

[alexcrichton]

This commit is the next step in fleshing out #12698 and following in the footsteps of #13361 to integrate the (implements ...) directive into bindgen!. Notably this world:

world foo {
    import a: thing;
    import b: thing;
}

will only generate a single trait for thing::Host as opposed to the previous 2 traits that were generated. This enables hosts to use the same trait implementation for both, or customize as appropriate.

Additionally the add_to_linker-generated methods for interfaces are now refactored to additionally have an add_to_linker_instance entrypoint. This new entrypoint takes a LinkerInstance instead of a Linker and fills in the provided instance directly. This is in-turn used to fill out a linker for the foo world above.

Notably, however, embedders can call add_to_linker_instance with distinct closures accessing different parts of T, the store's state, meaning that it's possible to have two distinct implementations for a and b above.

Many golden tests were updated with this new add_to_linker_instance method, and codegen/runtime tests were added for a simple world as well showcasing the bindings modes that are possible. The (rarely used) top-level world add_to_linker only uses a single trait implementation, but invoking add_to_linker_instance enables passing in two separate implementations.