This will be the first of a handful of refactoring PRs. The goal is to minimize our API surface and only export what needs to be exported. Hopefully none of these changes are too controversial, I think it should be quite straight-forward.

Starting with some modules: the command and elf modules contain implementation details which are used internally, and don't need to be exposed to users. Additionally, the error modules only contains one type which needs to be public, Error, which is now exported at the root of the package.

In addition to these changes, I have done some general cleanup and removed some unused code.

I have also decided that we do not need to document the modules/types gated behind the cli feature. This is all intended for use only for the cargo-espflash and espflash binaries, it's really of no concern to people using espflash as a library.