c#: Mechanical refactors

[jsturtevant]

There are a lot of changes here but no functional changes. Only a couple renames and mostly moved structs to their own modules to help with code organization and make it easier for some one new to understand the code. There are probably a lot of little improvements that could be made after this in an incremental way to reduce sharing across the modules but this is a good start. Each move/rename was done in a separate commit with cargo fmt/cargo check and some tests run against it.

I am suggesting these changes after having spent some time in the code. It took my awhile to understand the general flow and I think having the code organized in this way will help others understand the code faster.

[dicej]

It would be more efficient to make this return a &'static [&'static str] instead of allocating and populating a fresh new Vec every time it's called. Something like:

fn csharp_keywords() -> &'static [&'static str] {
    static ARRAY: &[&str] = &["abstract", ...];
    ARRAY
}

[jsturtevant]

Since that requires a bit more refactoring than just moving. I'll follow up on it with another PR