[ refactor ] Use smart consS instead of :# subst constructor.

consS (var n) $ wkS (n+1) IdS  optimizes to  wkS n IdS.
[ imports ] Always import Agda.Syntax.Common as Common.
[ Issue 1374 ] Fixed.  Invalid substitution was used in debug output.
[ refactor ] Always import Agda.Syntax.Common as Common