Add OptionalStanzaSet and OptionalStanzaMap

Makefile

...tall-solver/src/Distribution/Solver/Modular/Assignment.hs

...tall-solver/src/Distribution/Solver/Modular/Configured.hs

...ll-solver/src/Distribution/Solver/Types/OptionalStanza.hs

...all-solver/src/Distribution/Solver/Types/SolverPackage.hs

cabal-install/cabal-install.cabal.dev

cabal-install/cabal-install.cabal.zinza

cabal-install/src/Distribution/Client/Configure.hs

cabal-install/src/Distribution/Client/Install.hs

...l-install/src/Distribution/Client/ProjectOrchestration.hs

cabal-install/src/Distribution/Client/ProjectPlanning.hs

...-install/src/Distribution/Client/ProjectPlanning/Types.hs

cabal-install/src/Distribution/Client/SourceFiles.hs

...nstall/src/Distribution/Client/Types/ConfiguredPackage.hs

cabal-install/tests/UnitTests.hs

...tests/UnitTests/Distribution/Client/ArbitraryInstances.hs

...sts/UnitTests/Distribution/Solver/Types/OptionalStanza.hs