Refactor nestImplicTcS
Refactor nestImplicTcS

Simpler code, and simpler to understand.
No change in behaviour.

ezyang/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,olsner/ghc,sdiehl/ghc,snoyberg/ghc,mettekou/ghc,mettekou/ghc,shlevy/ghc,mettekou/ghc,snoyberg/ghc,olsner/ghc,olsner/ghc,mettekou/ghc,sdiehl/ghc,ezyang/ghc,snoyberg/ghc,ezyang/ghc,snoyberg/ghc,shlevy/ghc,sdiehl/ghc,olsner/ghc,olsner/ghc,snoyberg/ghc,ezyang/ghc,snoyberg/ghc,sdiehl/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,mettekou/ghc,mettekou/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,snoyberg/ghc,ezyang/ghc,mettekou/ghc