Make `impl_erased_set` public

[malobre]

Ideally, making the impl_erased_set public would allow users to create sets with arbitrary bounds, e.g:

impl_erased_set! {
    #[derive(Debug, Default, Clone)]
    pub struct MyErasedSet: Any + Clone + Debug;
}

Unfortunately, trait objects for multiple traits (i.e: dyn Any + Clone + Debug in this case) is currently not possible (see rust-lang/rfcs#2035).

[malobre]

Once rust-lang/rfcs#2035 is resolved this crate will only export impl_erased_set, which can be used to create erased sets tailored to the user needs.