Improve the sort-semantically for better idiomatic sort of generated Rust code

[amanjeev]

#1743 has one implementation of sorting, to get the resulting Rust code in a certain order. However, it can be better.

Input C/C++ Header

class Foo {
public:
  int foo;
  int get_foo();
};

class Bar {
public:
  int bar;
  int get_bar();
};

Bindgen Invocation

$ bindgen input.h --no-layout-tests --sort-semantically

Actual Results

/* automatically generated by rust-bindgen 0.60.1 */

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct Foo {
    pub foo: ::std::os::raw::c_int,
}
#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct Bar {
    pub bar: ::std::os::raw::c_int,
}
impl Foo {
    #[inline]
    pub unsafe fn get_foo(&mut self) -> ::std::os::raw::c_int {
        Foo_get_foo(self)
    }
}
impl Bar {
    #[inline]
    pub unsafe fn get_bar(&mut self) -> ::std::os::raw::c_int {
        Bar_get_bar(self)
    }
}
extern "C" {
    #[link_name = "\u{1}_ZN3Foo7get_fooEv"]
    pub fn Foo_get_foo(this: *mut Foo) -> ::std::os::raw::c_int;
}
extern "C" {
    #[link_name = "\u{1}_ZN3Bar7get_barEv"]
    pub fn Bar_get_bar(this: *mut Bar) -> ::std::os::raw::c_int;
}

and/or

Insert compilation errors generated when compiling the bindings with rustc here

Expected or rather Better Results

The order is actually, type followed by the type's impls and extern fns

/* automatically generated by rust-bindgen 0.60.1 */

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct Foo {
    pub foo: ::std::os::raw::c_int,
}
impl Foo {
    #[inline]
    pub unsafe fn get_foo(&mut self) -> ::std::os::raw::c_int {
        Foo_get_foo(self)
    }
}
extern "C" {
    #[link_name = "\u{1}_ZN3Foo7get_fooEv"]
    pub fn Foo_get_foo(this: *mut Foo) -> ::std::os::raw::c_int;
}
#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct Bar {
    pub bar: ::std::os::raw::c_int,
}
impl Bar {
    #[inline]
    pub unsafe fn get_bar(&mut self) -> ::std::os::raw::c_int {
        Bar_get_bar(self)
    }
}
extern "C" {
    #[link_name = "\u{1}_ZN3Bar7get_barEv"]
    pub fn Bar_get_bar(this: *mut Bar) -> ::std::os::raw::c_int;
}

[amanjeev]

closing this because there is a decent implicit sorting of the elements already.