pgrx requires careful updates for Postgres 19 due to Datum receiving ABI-breaking changes

I believe Postgres 19 (or perhaps 20, I haven't followed the patchset and mailing list super-closely) intends to use a struct which is always 8 bytes in size. These definitions are arguably more compatible conceptually with pgrx's notion of what a Datum is, but this means it will be passed differently than a 64-bit scalar in many C ABIs.