New layout for `Arguments` node to enhance performance. (and more so if possible)

[youknowone]

Currently the rust AST nodes layout is almost same as Python ast defined in Python side.

pub struct Arguments<R = TextRange> {
    pub range: OptionalRange<R>,
    pub posonlyargs: Vec<Arg<R>>,
    pub args: Vec<Arg<R>>,
    pub vararg: Option<Box<Arg<R>>>,
    pub kwonlyargs: Vec<Arg<R>>,
    pub kw_defaults: Vec<Expr<R>>,
    pub kwarg: Option<Box<Arg<R>>>,
    pub defaults: Vec<Expr<R>>,
}

Which is less parser-friendly than:

pub struct Arguments<R = TextRange> {
    range: ...,
    posonlyargs: Vec<Argument>,
    args: Vec<Argument>,
    vararg: Option<Box<Argument>>,
    kwonlyargs: Vec<Argument>,
    kwarg: Option<Box<Argument>>,
    type_comment: Option<String>,  // unparsed type_comment
    // no defaults and kw_defaults here
}

struct Argument {
    arg: Identifier,
    annotation: Option<Expr>,
    default: Option<Expr>,   // default is placed here unlike Arg
}

cc @MichaReiser

[MichaReiser]

I like that. It should also make it less error prone to inspect the AST