Restructure project to enable custom mavlink bindings generation

[GrahamDennis]

The goal of this PR is to enable generating Rust MAVLink bindings from custom MAVLink dialects in new crates. This is done by splitting the project into three crates:

• mavlink-core: core MAVLink types including TCP, UDP and serial connections
• mavlink-bindgen: a library & CLI for generating Rust bindings for MAVLink dialects
• mavlink: generated MAVLink bindings for the MAVLink dialects in the mavlink/mavlink repo

mavlink-bindgen can be used to create MAVLink bindings for Rust, and this is used from build.rs in the mavlink crate to create bindings from the standard MAVLink dialects in https://github.com/mavlink/mavlink.

The mavlink crate is intended to remain API-compatible with the previous structure.

[GrahamDennis]

This now passes all checks in my fork, so is ready for review @patrickelectric . The biggest change here is refactoring the crate into 3 crates in a single cargo workspace (which I think is required to supporting custom dialects). I'm happy to discuss alternative approaches or structures of a cargo workspace. Thanks!

[patrickelectric]

That looks awesome, thanks @GrahamDennis. I'll review asap.

[booo]

This change broke two links in the main README to the examples.