rustdoc typegen

[StuartHarris]

STACKED on top of #330

This is an experiment to see what it's like to 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 (or rather, for now, hook into the same serde_generate backend)
• generate serialisation and deserialisation code (also via serde_generate)

The simple_counter and counter examples in this PR have had their shared_types lib removed. To test them, use the crux_cli binary from this PR to generate the foreign types, e.g.:

cd examples/counter
../../target/debug/crux codegen --lib shared