Go Generic - experiments with Go 1.18 beta

Data structures:

• future.Future[T any] - Implementation of basic futures.
• iter.Iter[T any] and iter.IterCtx[T any] - lazy iterator pattern. Inspired by mtoohey31/iter, but using a single iterator function type.
• maps - A collection of helpful functions for working with maps.
• multimap.Multimap[K comparable, V any] - Convenience type for map[K]set.Set[V].
• pred package - A collection of predicate function generators.
• set.Set[E] - Convenience type for map[E]struct{}.