rustdoc typegen

[StuartHarris]

This is an experiment to see whether we can use rustdoc's JSON output to generate foreign types (in TS, Swift and Kotlin) for crux shared libs.

The JSON has all the information we need, so potentially we can use it to list all the types that implement crux_core::App, find their associated types and follow them to their leaves, even crossing crate boundaries (e.g. to crux_http). After we have gathered the relevant type information, we'd need to generate the foreign types, including any serialisation / deserialisation code).

• add a codegen subcommand to crux_cli
• invoke cargo doc and generate JSON output
• deserialise the output into rustdoc-types
• gather the relevant type information
• generate foreign types
• generate serialisation and deserialisation code