WATT version of proc_macro in cqrs-codegen crate

[tyranron]

To reduce compilation time and ensure safety via sandboxing, let's provide a watt version of our cqrs-codegen crate.

How it would work:

1. Create a no-watt feature for cqrs-codegen crate. So, by default the compilation will use watt, but a usual compilation may be opt-in if desirable.
2. Make all dependencies of cqrs-codegen crate optional behind no-watt feature.
3. Create a build.rs which compiles cqrs-codegen crate into src/codegen.wasm if it's absent.
4. Setup correct conditional compilation in src/lib.rs.
5. Re-export no-watt feature in cqrs crate as codegen/no-watt.
6. ...
7. PROFIT

Let's do it right after completing #2

[tyranron]

Consider rust-lang/cargo#4881 tricks for building codegen.wasm.

[ffuugoo]

Seen watt announcement few weeks ago. This is rad. 👍