Time to drop old-style (V0) functions?

Hi,

I'm wondering if it's not time for $subject:
- V0 causes confusion / weird crashes when PG_FUNCTION_INFO_V1 was forgotten
- They have us keep weird hacks around just for the sake of testing V0
- they actually cost performance, because we have to zero initialize Datums, even if the corresponding isnull marker is
set.
- they allow to call arbitrary functions pretty easily

I don't see any reason to keep them around. If seriously doubt anybody
is using them seriously in anything but error.

Greetings,

Andres Freund