Merge branch 'set-interface' into '1.0.x'

src/BaseTypes.php

src/DataStructure/CallableImmutableSequence.php

src/DataStructure/FloatImmutableSequence.php

src/DataStructure/IntImmutableSequence.php

src/DataStructure/ObjectImmutableSequence.php

src/DataStructure/Set.php

src/DataStructure/SetInterface.php

src/DataStructure/StringImmutableSequence.php

src/Drush/Generators/templates/map-immutable.php.twig

src/Drush/Generators/templates/typed-sequence.php.twig