[ test suite ] Deleted unused module Imports.Coinduction.

Common.Coinduction is used instead.
[ test ] for ClashingDefinition when open public.
[ test ] for ClashingModule when open public.
[ refactor ] A lens for scopeNameSpaces.
[ clean ] lens for ScopeInfo alreay defined in TC.Monad.State.
[ refactor ] Rewrote some lenses in pointfree style.
[ refactor ] ...ed checkClashes using when and unless.
[ debug ] Renamed debug tag tc.constr.findInScope to tc.instance.
[ refactor ] renamed modifyScopeLocals to updateScopeLocals.
[ test ] for shadowing by let open.
[ issue 1266 ] test cases
[ new ] Utils.AssocList missing operations of association lists
[ refactor ] Using AssocList.

Note: this refactoring assumes that the NameSpaces were indeed
association lists, i.e., each NameSpaceId is only mapped to one
NameSpace.  (Seems to be true and intended.)
[ refactor ] More lenses in Scope ops.
[ refactor ] Renamed ns to acc (NameSpaceId).
[ issue 1266 ] Fixed by letting imports shadow local vars.
[ clean ] Removed a line of dead code.
Merge branch 'maint-2.4.2'