Allow overriding generated python method names

[programmerjake]

Allow code like:

#[pymethods]
impl MyType {
    // public API for Python
    #[name = "my_method"]
    #[args(arg2 = "None")]
    fn my_method_python(&self, arg1: i32, arg2: Option<i32>) {
        match arg2 {
            Some(arg2) => self.my_method_with_arg2(arg1, arg2),
            None => self.my_method(arg1),
        }
    }
}

impl MyType {
    // public API for Rust
    pub fn my_method(&self, arg1: i32) {
        unimplemented!()
    }
    pub fn my_method_with_arg2(&self, arg1: i32, arg2: i32) {
        unimplemented!()
    }
}

my_method is accessible from Python:

a = MyType()
a.my_method(1)
a.my_method(1, 2)

This is useful to allow the Rust and Python APIs for a type to differ somewhat due to different conventions between Python and Rust.

There is already precedent for this kind of renaming: #[pyclass(name = MyType)]