MacEager is a MacResult implementation for the common case where you've already
built each form of AST that you might return.
Fixes#17637. Based on #18814.
This is a [breaking-change] for syntax extensions:
* MacExpr::new becomes MacEager::expr.
* MacPat::new becomes MacEager::pat.
* MacItems::new becomes MacEager::items. It takes a SmallVector directly,
not an iterator.