feat: Improve settings macros to handle default

[reneca]

Since settings and proc_settings macros are adding members to a struct,when you try to implement Default trait, you have an issue because you also need to specify those extra members.

The feat is here to do it seamlessly. You only have to use the macro when you implement the Default trait and forget about extra members.

See doc example in the change to know more.

[oschijns]

#[proc_settings] does not prevent manual implementation of the Default trait.
#[proc_settings] provide appropriate compiler error messages when used incorrectly.