[stdlib] Refactor external_call() and inline _LITRefPackHelper into VariadicPack

[martinvuyk]

Refactor external_call() and inline most of _LITRefPackHelper into VariadicPack, leave _LITRefPackHelper itself intact since I found no way to inline the AddressSpace parameter into VariadicPack.

[JoeLoser]

!sync

[JoeLoser]

CC: @ConnorGray as you have an internal PR up for consolidating _LITRefPackHelper and VariadicPack from other work you were doing.

[ConnorGray]

!sync

[martinvuyk]

@ConnorGray fixed all conflicts, this became mostly a cleanup PR

[ConnorGray]

Hi Martin, I appreciate you taking the time to fix up the conflicts. However, as Joe mentioned, coincidentally I was finishing work in this same area recently, and ended up performing a similar combination of _LITRefPackHelper and VariadicPack (with the addition that, with some advise from Chris L., we realized we could remove the address space parameter entirely for the time being, allowing a full removal of _LITRefPackHelper).

I'm sorry for stepping on your toes a bit here, that's largely my fault for not being as aware of your prior work in this area as I should have been 😕

With that in mind, this PR still has some nice doc comment additions and improvements, so I'm going to proceed with fixing up the conflicts and making sure that work is still incorporated.

Thank you for all the work you've been doing cleaning up some of these warts in the standard library, it's not always the flashiest work, but very important for having a good foundation and keeping the stdlib healthy. My apologies again Martin 😌

[ConnorGray]

!sync

[JoeLoser]

@martinvuyk do you mind rebasing this on top of @ConnorGray's recent changes in this area/the latest nightly? We'd love to incorporate the doc improvements here. Thanks!

[martinvuyk]

@JoeLoser done