Add goal for expansion-time evaluation

[tmandry]

This is based on the hackmd by @oli-obk: https://hackmd.io/El2IX0c4QqaWr6494MbEVQ

Rendered

[oli-obk]

This is a direct ask for reviews or at least vibe checks from @petrochenkov

[joshtriplett]

I'm a poor substitute for @petrochenkov at best, but I've gotten mildly familiar with some of it as a result of the macro work and I'm happy to provide a second set of eyes where it might help.

[joshtriplett]

Awesome. I'm planning on submitting a goal for macros; I'd planned to hold back from macro fn and mark it as "future work", but if this goal gets accepted then I'd love to align with it and see if I can make macro fn work this year.

I'm also happy to help with this goal, as needed. That includes sponsoring any potential lang experiments needed to try exposing this functionality.