add an `impl_eq!` macro that implements `__richcmp__`

[MarquessV]

For types that implement PartialEq/Eq, Python's __richcmp__ dunder method is easily defined as:

#[pymethods]
impl PyType {
    pub fn __richcmp__(&self, py: Python<'_>, other: &Self, op: CompareOp) -> PyObject {
        match op {
            CompareOp::Eq => (self.as_inner() == other.as_inner()).into_py(py),
            _ => py.NotImplemented(),
        }
    }
}

An impl_eq! macro would be a nice addition to handle this case. The overlap with impl_compare! would have to be well documented to avoid confusion.