read cargo configs with/without .toml extension + make public API clearer

[chrismooredev]

The goal for my series of commits is to consolidate repeated functionality and make this crate easier to use for those using it's public Rust API.

commit ff1e158:

• prevents cargo_edit::registry::cargo_home from unnecessarily getting $HOME, or checking UTF8 compliance.

commit 03f6d74: introduce new structs to drive a new public, programmable API

• supports reading both bare .cargo/config and .cargo/config.toml files
• added documentation
• puts some of the public API into new RegistryReq and RegistryIndex structs, and attaches many of the freestanding functions in cargo_edit::registry into those.
  • RegistryReq notably allows a new user of the public API to more easily figure out the latest dependency of a crate without supplying a project directory.

commit 1c986a6:

• rolls back changed public signatures, to forward to the new functions, with deprecation warnings
• moves implementations to their respective RegistryReq and RegistryIndex structs

commit 933ebfe:

• bakes in the generated binary paths (cargo-add/cargo-rm/cargo-update) for testing purposes
• also enables testing this crate within a cargo workspace, as the target/ dir is not in the expected crate root or CWD location for those.

I plan to do a major bump once I am done editing the public API, if that is welcomed :)