tc: term: track some implicit in annotations being lost
tc: term: track some implicit in annotations being lost

We were checking the types for each let rec, but not tracking the
implicits that they might have. This is needed, as the many repros in
 #1443 show.

FStarLang/FStar,A-Manning/FStar,A-Manning/FStar,A-Manning/FStar,FStarLang/FStar,FStarLang/FStar