Fix Trac #3813: unused variables in GHCi bindings

Fix Trac #3813: unused variables in GHCi bindings

In a GHCi stmt we don't want to report unused variables,
because we don't know the scope of the binding, eg

	Prelude> x <- blah

Fixing this needed a little more info about the context of the stmt,
thus the new constructor GhciStmt in the HsStmtContext type.