Add support for GATs with trait bounds and where clauses

[Dzejkop]

Hey! First of all great crate and lovely code - it's been a breeze figuring this out.

Currently the following code:

#[auto_impl(Arc, Box, Rc, &, &mut)]
trait HasAssociatedTypeWithBoundsAndWhereClause {
    type AssociatedType<'a, T>
    where
        T: From<usize> + 'a;
}

fails to compile because the macro doesn't generate the correct trait bounds and/or where clause on the associated type.

This PR fixes it by splitting the generics of an TraitItemType with Generics::split_for_impl - doesn't affect any other cases.

[KodrAus]

Thanks @Dzejkop! This looks good to me