Add crates for wasmtime integration and a cli interface

[pchickey]

This is a pretty big PR towards making component-init usable standalone:

• Introduce a new crate component-init-wasmtime that provides a wasmtime-based Invoker. Right now this Invoker only supports trapping on calls to any imports, will add WASI support in a follow-up PR
• Introduce a new crate component-init-cli that provides a CLI interface to component-init-wasmtime. Just adds the clap dep here.
• redesign component-init-get-memory to no longer try to use a __stack_pointer global, which only exists in the way component-init expects in components crated with wasm-tools component link. Instead, a new allocation strategy is used that ignores all contents of the module and just calls mem.grow, which it uses to store the return value pointing to the previous full range of memory.
• add tests for component-init-wasmtime: two handwritten wat components show it handles initialization of memory and a mut global, and one rust guest shows it works for a "real" component
• add a CI job that runs the above tests, checks formatting, and checks docs. You can see that CI runs here: Pch/wasmtime and cli pchickey/component-init#1

In addition to the newly written tests, I tested my work against componentize-py and posted the following PR to update componentize-py to this version of component-init bytecodealliance/componentize-py#159. That PR should wait to land until we publish component-init to crates.io, in my opinion.

[dicej]

LGTM; thanks so much for doing this! Just a couple of inline comments, both optional.

[dicej]

I'd be in favor of running clippy here unless there's a good reason not to.

[pchickey]

Good idea! 10a0482

[dicej]

Nit: using AtomicBool here would be simpler and require less explanation.

[pchickey]

Yep 24fb6dd