[PPC0030] - Clarifications on `use overload` interaction

The PPC document doesn't actually talk about use overload at all.

In the course of implementing it (https://github.com/Perl/perl5/pull/24304), currently I have fully created an overload category for the equ and === operators, allowing a class to use overload equ => ... to specify its overloading behaviour here.

But I wonder if this should be considered correct, or not. We say that the equ operator's behaviour can be explained in terms of some defined checks and the eq operator. While we allow the eq operator to be overloaded, we don't allow defined to be.

I wonder if therefore, we should say that equ (and related) are not independently overloadable, but instead are just implemented in exactly these terms - a defined check first that cannot be overridden, followed by a possibly-overloaded eq check.

Start by reading the PPC document and the linked perl5 pull request, then review how use overload currently treats equ and ===. Done means the interaction is resolved as a clear specification decision and the PPC document explains whether these operators are independently overloadable or defined through defined and eq.