Refactor cargo-component internals and introduce proc-macro.

[peterhuene]

This PR includes a very large refactoring to the internals of cargo-component and how it works.

The biggest change is that cargo-component no longer generates an external bindings crate that is automatically added as a dependency to a user's component project. The motivation behind this change is that support for resources from the component model will preclude the ability to reference types from the user's crate because the export! macro is being removed.

Instead, component projects will need to reference the cargo-component-bindings crate to invoke the generate! proc-macro. This macro will expand to the bindings code based on the target world encoded from dependency resolution performed by cargo-component.

By default, the generate! macro will assume that the world / all exported interfaces are implemented by a type named Component, however that can be changed with an argument to the macro.

With the removal of the need for an external bindings crate, cargo-component no longer has a concrete reason to directly depend on the cargo crate as a dependency. Instead, it now spawns cargo for all subcommands that aren't built-in to cargo-component.

For this to function, cargo-component detects certain cargo options that may alter output (--verbose, --quiet, etc.) or what gets built (--manifest-path, --target, etc.). Before any unrecognized commands are passed to cargo, cargo-component will update the encoding of target worlds for bindings allowing commands that analyze or build the source code to work.

Additionally, cargo-component will recognize certain cargo subcommands that have outputs that need to be componentized.

As a result of these changes, about 150 crate dependencies were removed, the cargo-component executable shrunk about 12 MiB in size, and commands such as cargo component expand now "just work" without cargo-component having to provide its own implementation.

All dependencies, and the WASI adapters, are updated as part of these changes.

Note: the test command still isn't implemented.

Closes #97.
Closes #91.

[peterhuene]

Additionally, this PR also refactors some of the cargo-component internals into a "core" crate.

This will allow for sharing implementation with cargo-component and the wit tool from https://github.com/peterhuene/wit (which I plan to merge into this repo shortly).

[sunfishcode]

I'm not very familiar with the registry parts, but overall this looks good! My main concern is just how we can make the bindings module accessible to other modules:

[peterhuene]

@sunfishcode thanks for looking this huge change over!