Deduplicate names of overloaded virtual methods

[SimonSapin]

Test case:

class Stream {
    public:
    virtual void write(char* string);
    virtual void write(void* buffer, unsigned int size);
};

Code generated by bindgen 1e26a8a:

#[repr(C)]
#[derive(Debug, Copy)]
pub struct Struct_Stream {
    pub _vftable: *const _vftable_Struct_Stream,
}
#[repr(C)]
pub struct _vftable_Struct_Stream {
    pub write: unsafe extern "C" fn(this: *mut ::std::os::raw::c_void,
                                    string: *mut ::std::os::raw::c_char),
    pub write: unsafe extern "C" fn(this: *mut ::std::os::raw::c_void,
                                    buffer: *mut ::std::os::raw::c_void,
                                    size: ::std::os::raw::c_uint),
}
impl ::std::clone::Clone for Struct_Stream {
    fn clone(&self) -> Self { *self }
}
#[test]
fn bindgen_test_layout_Struct_Stream() {
    assert_eq!(::std::mem::size_of::<Struct_Stream>() , 8usize);
    assert_eq!(::std::mem::align_of::<Struct_Stream>() , 8usize);
}

Struct fields with the same name are an error in Rust. Similar code fails with:

    --> src/bindings.rs:1987:5
     |
1985 |     pub write: unsafe extern "C" fn(this: *mut ::std::os::raw::c_void, b: u8)
     |     - `write` first declared here
1986 |                    -> usize,
1987 |     pub write: unsafe extern "C" fn(this: *mut ::std::os::raw::c_void,
     |     ^ field already declared

The field names should probably be deduplicated by appending an integer, like for names of non-virtual methods.

CC @emilio @nox