Provide macro in method builtins for better object value extraction

[phorward]

Many methods in Tokay are currently defined exemplary this way:

tokay_method!("dict_len(dict)", {
    let dict = dict.borrow();

    if let Some(dict) = dict.object::<Dict>() {
        Ok(RefValue::from(dict.len()))
    } else {
        Err(Error::from(format!(
            "{} only accepts '{}' as parameter, not '{}'",
            __function,
            "dict",
            dict.name()
        )))
    }
});

The only relevant code implementing the method's functionality is

Ok(RefValue::from(dict.len()))

This could be replaced by a much shorter, nice macro call that could look as something like this:

tokay_method!("dict_len(dict)", {
    with_object!(dict, Dict, {
         Ok(RefValue::from(dict.len()))
    })
});

The following macros are required:

• with_object - borrows an object for read, throws an error when given RefValue is not the given object
• with_object_mut - borrows an object for write, throws an error when given RefValue is not the given object

[phorward]

After some tests, the initial idea of this issue isn't possible due Rusts macro hygiene feature.

[phorward]

Alternatively to a macro, two functions

• RefValue::with_object<T>(Fn<&T> -> Result<R, Err<Error>>) -> Result<R, Err<Error>>
• RefValue::with_object_mut<T>(FnMut<&mut T> -> Result<R, Err<Error>>) -> Result<R, Err<Error>>

could be made available.. They borrow the value, cast to the particular object and run a function. In case the object cannot be borrowed or the type doesn't match, an error is generated and thrown.