Add implementation of TaskSeq.except/exceptOfSeq

[abelbraaksma]

This implements the following, in line with Seq.except. I'm considering adding a TaskSeq.remove, as it'd be a much lighter implementation and users may not always need set-except operation style.

This adds:

module TaskSeq =
    val except<'T when 'T: equality> : itemsToExclude: taskSeq<'T> -> source: taskSeq<'T> -> taskSeq<'T>
    val exceptOfSeq<'T when 'T: equality> : itemsToExclude: seq<'T> -> source: taskSeq<'T> -> taskSeq<'T>

@dsyme I'm not 100% sure this r/w lock approach is ideal (as I have to use it synchronously), I'm considering either a semaphore or adding an F# version of Stephen Cleary's AsyncReaderWriterLock (itself based on Stephen Toub's) implementation, which may be useful to expose as well somehow.

A similar comment about using a r/w lock was mentioned in some spots in AsyncSeq.

https://github.com/StephenCleary/AsyncEx/blob/8a73d0467d40ca41f9f9cf827c7a35702243abb8/src/Nito.AsyncEx.Coordination/AsyncReaderWriterLock.cs#L16

[abelbraaksma]

I asked around about this approach. Some people recommended removing the try/finally as hashSet.Add doesn't raise. This is probably correct, but until I know more, I'll leave that in, as entering a protected region is quite cheap in .NET.