Adds a setting to force some types to always be included in the emitted types

[xanathar]

This is useful to support, for example, enum types whose values are significant but whose type is not explicitly used in the functions prototypes.

For example in this code:

/// Returns the id of an opened object, or - if negative - an error code
#[no_mangle]
pub extern "C" fn open_object() -> i64 {
...
}

#[repr(i64)]
pub enum ErrorCode {
     InvalidState = -1,
     BadMood = -2,
     NotAGoodDay = -3,
}

ErrorCode would not be emitted in the generated bindings because it doesn't appear in open_object prototype even if it is a potential return type.

[tvandinther]

This is a great feature. I've had to create dummy functions utilising these types to have them exported. With this PR, how does the usage change?

[xanathar]

Good point, I forgot to add an example/update the README.

Give me a day or two, I'll send an update.

But essentially its as simple as:

    csbindgen::Builder::default()
        .input_extern_file("myfile.rs");
        .csharp_dll_name("mymodule.dll")
        .csharp_namespace("MyNamespace")
        .always_included_types(["ThisType", "ThisOtherType"])
        .generate_csharp_file("csharp/mybindings.g.cs")
        .unwrap();

[hadashiA]

👍 Thanks !