Split the proc macro to dependency crate

[idanarye]

The TypedBuidler custom derive proc macro generates some helpers. In current master there are {BuilderType}_Optional and {BuilderType}_core, and #30 is adding some bottom types ({BuilderType}_Error_Missing_required_field_{field_name} and {BuilderType}_Error_Repeated_field_{field_name})

These things are weirdly mangled and repeated for every struct with a typed builder even though they are exactly the same - it would be better if we could just put them in the typed builder crate, with legal Rust names, and just reuse them. The problem is that proc macro crates are not allowed to export anything that's not a proc macro.

The solution: split the proc macro to a new crate - say typed-builder-procmacro - and have typed-builder re-export it. This is the common solution for this problem in Rust-2018 where macros can be imported/exported like regular symbols.

[idanarye]

Actually, I think I'll keep {BuilderType}_Error_Repeated_field_{field_name} - at least for now - because it gets into the build error. {BuilderType}_Error_Missing_required_field_{field_name} doesn't show in the error message so I'll replace it once I split the crate.

[mo8it]

@idanarye May I do this?

[idanarye]

Sure