Use macros for module declarations on registry

[k-nasa]

Motivation

ref: #1009

This is code refactoring. Currently, there is a lot of code that declares modules as follows:

#[cfg(feature = "registry")]
#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
pub use registry::Registry;

#[cfg(feature = "registry")]
#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
pub fn registry() -> Registry {
    Registry::default()
}

It is verbose to write a module declaration multiple times using the feature attribute. Also, if the definition for the same feature spans multiple places, it will be a little harder to read the code.

Solution

You can combine features attributes in one place by writing as follows. It also eliminates the need to write the same code over and over again.

cfg_feature!("feature" ,{
    pub use registry::Registry;

    pub fn registry() -> Registry {
        Registry::default()
    }
});

[davidbarsky]

Thanks! Can you format this PR to pass CI?

[k-nasa]

I fix it!

[davidbarsky]

Thank you, and sorry for the delay! I was on vacation last week.