Add async Linq extensions for `AsyncEnumerable`

It won't just be copying the regular Linq extensions like Chunk that return newly allocated collections. Will need to be clever about exposing temporary collections so they can be pooled after use.

[x] Aggregate
[x] AggregateBy
[x] All
[x] Any
[x] Append
[x] Average
[ ] ~~Cast~~ (Not including since we don't support non-generic AsyncEnumerable, and we can't have covariant structs, and C# doesn't allow generic extension classes.)
[x] Chunk
[x] Concat
[x] Contains
[x] Count
[x] CountBy
[x] DefaultIfEmpty
[x] Distinct
[x] DistinctBy
[x] ElementAt
[x] ElementAtOrDefault
[x] Except
[x] ExceptBy
[x] First
[x] FirstOrDefault
[x] GroupBy
[x] GroupJoin
[x] Index
[x] Intersect
[x] IntersectBy
[x] Join
[x] Last
[x] LastOrDefault
[x] LongCount
[x] Max
[x] MaxBy
[x] Min
[x] MinBy
[ ] ~~OfType~~ (Not including for the same reasons as Cast.)
[x] Order
[x] OrderBy
[x] OrderByDescending
[x] OrderDescending
[x] Prepend
[x] Reverse
[x] Select
[x] SelectMany
[x] SequenceEqual
[x] Single
[x] SingleOrDefault
[x] Skip
[x] SkipLast
[x] SkipWhile
[x] Sum
[x] Take
[x] TakeLast
[x] TakeWhile
[x] ThenBy
[x] ThenByDescending
[x] ToArray
[x] ToDictionary
[x] ToHashSet
[x] ToList
[x] ToLookup
[x] Union
[x] UnionBy
[x] Where
[x] Zip

timcassell / ProtoPromise

Add async Linq extensions for `AsyncEnumerable` #294