API Proposal: IAsyncEnumerable intergrate with LINQ

[hez2010]

For example:

IAsyncEnumerable<T> xxxx;

var result = await xxxx.Where(i=> .....).OrderBy(i=> ...).AsAsync();
var result = await from async c in xxxx where .... order by ..... asc;

[benaadams]

I think Reactive Extensions provides for this in v4 https://github.com/dotnet/reactive/tree/master/AsyncRx.NET

/cc @onovotny

[GalaxiaGuy]

I think this was recently discussed: https://github.com/dotnet/corefx/issues/36545

The conclusion was:
https://github.com/dotnet/reactive/tree/master/Ix.NET/Source/System.Linq.Async

[stephentoub]

Closing as dup of https://github.com/dotnet/corefx/issues/36545.

[stephentoub]

#111685 has added System.Linq.AsyncEnumerable.