Remove FnOnce requirement

[piccoloser]

Changes in This PR

Changes to Config::load_or_default()

• No longer requires a FnOnce() -> Config as its second argument
• Now accepts a Config instead, without a closure

// Before (Disliked by Clippy)
Config::load_or_default(CONFIG_INI, || {
    return sectioned_defaults! {
    // ...
    };
})

The issue with the original implementation was that clippy would recommend removing the unneeded return statement. Doing so, however, would lead to errors because sectioned_defaults! and general_defaults! create configurations by running Config::builder().set("...", "...").build(); without returning anything. The only other option that wouldn't be flagged by Clippy was to format your closure this way:

// Before (Accepted by Clippy, disliked by cargo fmt)
Config::load_or_default(CONFIG_INI, || sectioned_defaults! {
    // ...
})

... And then running cargo fmt would change it to this:

// Before (Broken after cargo fmt)
Config::load_or_default(CONFIG_INI, || {
    sectioned_defaults! {
        // ...
    }
})

This adds curly braces around the macro and breaks the function execution immediately.

With the new implementation, you can simply do this:

// New implementation
Config::load_or_default(CONFIG_INI, sectioned_defaults! {
        // ...
})

[piccoloser]

This PR resolves #3