Rollup merge of #22875 - kmcallister:maceager, r=sfackler

 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.

r? @sfackler