Accessing argument metadata from tclass_field

Currently there doesn't seem to be a way to access argument metadata on an extern function.

extern class Foo {
    extern function bar(@:cpp.Marshal(c_string) s:String):Void;
}

In the above case there is no way to access the @:cpp.Marshal metadata from a tclass_field. In non extern functions you can inspect the cf_expr TFunction but for externs there is no expression.

I think I've found the place in typeloadFields which types functions but I'm not sure what a good approach is, append the arguments meta to cf_meta and key it by argument name somehow? Add a new cf_argument_meta field?

End goal of this to to have the compiler generate glue code for converting to and from common haxe and c++ types by annotating arguments, it would greatly simplify things being able to look at all extern classes up front as opposed to accumulating a list of all found functions while iterating over the ast.

Start in typeloadFields, where extern functions are typed, and inspect how tclass_field stores cf_meta, cf_expr, and TFunction data for non-extern functions. Compare the available representations and define a way for argument metadata such as @:cpp.Marshal to remain accessible from extern fields; done means compiler code can inspect that metadata while processing extern classes.