Update 'inactive_overload_cache' key

[BetsyMcPhail]

Instead of just the function name, use __qualname__.name.

Solves issue #1922

@wjakob

[EricCousineau-TRI]

@YannickJadoul @rwgk I was updating our fork, and forgot that we hadn't landed this (see linked issue for more deets).

Is this a candidate for 2.6.0?

[EricCousineau-TRI]

Came across this when crafting: RobotLocomotion#47

[YannickJadoul]

This should probably use detail::get_fully_qualified_tp_name (from #2520), but apart from that, a quick look says "yes, I think so?"

[YannickJadoul]

At the same time, it's still quite late to add, though? :-/ @henryiii?

[YannickJadoul]

Also, what if the new class has exactly the same name as the old one?

[bstaletic]

Also, also, the changes to the hash function are really wrong. You don't want to combine hashes with just an xor.

[henryiii]

We can work on it for 2.7 - I think it's too late for 2.6 for anything with a chance of breaking.

[EricCousineau-TRI]

Sounds good! We'll address this for release 2.7.0, not 2.6.0 - thanks!

[EricCousineau-TRI]

Per Slack convo w/ Betsy and @jamiesnape, I will re-open this as author (but maintain Betsy's authorship + traceability). Then I'll update it for latest master, and see if we can sneak it into future releases.

[EricCousineau-TRI]

Closed in lieu of superseding PR, #2772