Add AsRef and Borrow for generate_mut_trait_impl

[A4-Tacks]

• AsRef -> AsMut
• Borrow -> BorrowMut

Example

//- minicore: as_ref
struct Foo(i32);

impl<T> core::convert::AsRef$0<i32> for Foo {
    fn as_ref(&self) -> &i32 {
        &self.0
    }
}

->

struct Foo(i32);

$0impl<T> core::convert::AsMut<i32> for Foo {
    fn as_mut(&mut self) -> &mut i32 {
        &self.0
    }
}

impl<T> core::convert::AsRef<i32> for Foo {
    fn as_ref(&self) -> &i32 {
        &self.0
    }
}