Draw the rest of the ̶o̶w̶l̶ new ABI-handling framework

[workingjubilee]

brain recently tied together the rest of the connections necessary for implementing all of https://github.com/pgcentralfoundation/pgrx/issues/1337 and thus finishing out the fixes that ultimately address the soundness issues revealed by the Spi bugs we found.

• [x] BorrowDatum
  • better support zero-allocation datum conversion
  • [x] prototype
  • [ ] identify unresolved design questions
• [ ] DetoastDatum
  • support datums with potentially very complex unboxing behavior
  • [ ] prototype
  • [ ] identify unresolved design questions
• [ ] MemCx
  • need significant API expansion
  • [x] prototype
  • [ ] API expansion to make it actually usable
  • [x] https://github.com/pgcentralfoundation/pgrx/pull/1737
  • [ ] need an answer for stable, pgrx-specific types
• [x] Transition to new argument/return strategies that enable checking lifetimes
  • [x] surface lifetimes into internal wrapper signatures so arguments/returns can use them
  • https://github.com/pgcentralfoundation/pgrx/pull/1461
  • https://github.com/pgcentralfoundation/pgrx/pull/1462
  • https://github.com/pgcentralfoundation/pgrx/pull/1463
  • https://github.com/pgcentralfoundation/pgrx/pull/1476
  • https://github.com/pgcentralfoundation/pgrx/pull/1724
  • [x] replacement for pg_getarg with a different signature for the new ABI, probably https://github.com/pgcentralfoundation/pgrx/issues/1428
  • [x] try playing type-tetris with traits and blanket impls to implement returns via traits ( became https://github.com/pgcentralfoundation/pgrx/pull/1701 )
  • [x] try playing type-tetris with traits and blanket impls to implement argument-passing via traits ( became https://github.com/pgcentralfoundation/pgrx/pull/1731 )
  • many unresolved design questions
• [ ] Retire FromDatum, once replacements are in place:
  • [x] ArgAbi
  • [x] BorrowDatum
  • [ ] DetoastDatum
• [ ] Spi: https://github.com/pgcentralfoundation/pgrx/issues/1483

[workingjubilee]

We still need to introduce some transition code here for cases where it isn't as obvious a breakage, but implementing the new RetAbi was more successful in preserving functionality than I expected, and most of the errors produced are straightforward type errors now, so I'm making more hard breaks.

[workingjubilee]

The first piece of this has slid into place, as #1731 now makes it possible, in conjunction with #1701, to have a single pg_extern function be lifetime-checked on its arguments and return types.