pyfn: deprecate name argument

[davidhewitt]

This PR makes the second "name" argument to #[pyfn] both optional and deprecated in favour of #[pyo3(name = "...")]. This is part of #1601

This has the nice effect that now #[pyfunction] and #[pyfn(m)] take equivalent options, so I have rewritten the documentation to primarily talk about #[pyfunction], and #[pyfn] is introduced as syntax sugar.

Before:

#[pymodule]
fn rust2py(py: Python, m: &PyModule) -> PyResult<()> {

    #[pyfn(m, "sum_as_string")]
    fn sum_as_string() { }

    #[pyfn(m, "custom_name")
    fn foobar_to_rename() { }

    Ok(())
}

After:

#[pymodule]
fn rust2py(py: Python, m: &PyModule) -> PyResult<()> {

    #[pyfn(m)]
    fn sum_as_string() { }

    #[pyfn(m)]
    #[pyo3(name = "custom_name"]
    fn foobar_to_rename() { }

    Ok(())
}

[davidhewitt]

If anyone's got any opinions on this, or wants to review the code, please let me know before the end of the weekend. Else I'll assume this is all fine and merge it before continuing with the rest of the items in #1601

[davidhewitt]

Merging as promised! If there is any feedback on this I can always make follow-up PRs (or revert).