Candid to Rust binding

There is no one-to-one mapping between Candid and Rust types.

• A candid type can map to multiple Rust types. For example, vec record { text; int } can map to Vec<(String, Int)>, HashMap<&str, Int>, or even &[Box<(Cow<str>, Arc<i128>)>].
• The fundamental difference between Candid typing and Rust is structural typing vs nominal typing. That is, when you create a struct/enum type in Rust, you have to give the type a name, which doesn’t necessarily exist in the did file, or may not the best name you want. There’s also the question of what traits to derive for each struct.

As a starting point, we can generate a simple binding with default types and let the users to adjust the binding by hand. To automate the customization, we can design a builder similar to https://docs.rs/prost-build/0.7.0/prost_build/struct.Config.html

TODO:

• design for config struct
• handle management canister for agent target
• detect if renaming collapse names
• non-blocking call for agent
• tests for golden files

Start by reviewing the proposed default Candid-to-Rust bindings and the TODO list, beginning with the config struct design. The issue names no files or entry points; completion would require resolving the listed design, agent, naming, non-blocking call, and golden-file testing work.