Fix unwind passing of REDUCE

This fixes CureCode issue #1760: "Unwind functions don't propagate from
REDUCE".
Fix unwind passing of WHILE

This fixes CureCode issue #1519: "Unwind functions don't propagate from
WHILE condition block".
Fix unwind passing of set-word! and SET

This fixes CureCode issue #1515: "Unwind values are assignable with
set-word expressions, SET or SET/any".
Fix unwind passing of functions accepting error!

This fixes CureCode issue #1509: "Unwind functions don't propagate when
called in function arg expression that accepts error! values"
Fix unwind passing of APPLY
Fix unwind passing of set-path!
Fix unwind passing of AJOIN
Add comments to point to where unwinds are handled
Merge earl's fix-unwind-passing into community. Tested successfully in 0.3.1, 0.3.3, 0.4.4, 0.4.40, 0.13.1. No regressions, lots of progressions including crash fixes.