GROOVY-6920: Add disjoint, minus and toSpreadMap to Iterable

GROOVY-6920: Add disjoint, minus and toSpreadMap to Iterable

Added these methods to Iterable.
disjoint(Iterable)
minus(Object)
minus(Iterable)
toSpreadMap()

Added this method to Collection.
minus(Collection)

I forget to add those methods in https://github.com/groovy/groovy-core/pull/444 and https://github.com/groovy/groovy-core/pull/449 .