This commit refactors the subtype checks in the validator type information to
support comparing types from different type lists.

This will allow tools to validate different components and be able to easily
perform subtype checks from the disjoint type lists.

This commit exposes the `exports` utility method from the validation instance
types.

This commit extends the validator type information to include getting the
entity types for imports and exports.

This commit implements printing components that alias type exports from
component instances.

This commit implements `wasm-compose`, a tool for composing WebAssembly
components from other components.

Right now the tool is pretty simple: you specify a list of component imports,
specify how they get instantiated, and which of the instances gets its exports
exported from the composed component.

`wasm-compose` operates directly on WebAssembly components and has no
dependencies on `wit` or `wit-bindgen`.

This commit normalizes the path separator for error baselines in the tests for
`wasm-compose`.

It also normalizes a platform-specific error message.

This commit removes the `wasm-compose` standalone binary and moves its
functionality to the `wasm-tools compose` subcommand.

It also updates the example README to use `wasm-tools compose`.

This tweaked a bunch of the error messages and made missing/incompatible
exports for instances a little clearer.

This commit changes how the composition configuration format is defined.

It replaces `imports` with `components` and makes embedding components the
default.

The simplified format will hopefully make more sense.

In addition, the explainer for the configuration file format has been split out
from `README.md` into `CONFIG.md`.

This commit removes support for TOML as the configuration file format and just
supports YAML.

This commit updates the example component `Cargo.toml` formats for
`wasm-compose` to include changes to the latest `cargo component`.

This commit replaces the example with a sillier, but slightly more complex,
example using lists in the interfaces (hot off the presses).

This commit updates the composition demo to include a simple HTTP service that
uses a configurable backend to talk to.

The backend in the demo expects a `text/plain` body and echoes the body in the
response.

With a simple change to how the component gets composed, a middleware component
can be inserted between the service and the backend components; it is
responsible for gzip compressing the response from the backend.