Packing records?

From call today: can we add some syntax like `pack {x=e;y=f} as r`, that elaborates to something like `let r = {x=e; y=f}; rewrite each e as r.x; rewrite f as r.y;` to comfortably use records? Or would it be enough to define a packing function for each given type? The latter alternative would not do any rewrite in the frame.

No files, tests, or entry points are named. First compare the proposed `pack {x=e;y=f} as r` elaboration with the alternative per-type packing function, then establish the intended syntax, rewrite behavior, and acceptance criteria before identifying the implementation and tests.

Written by the indexing model from the issue text.