Add TaskSeq.fold and TaskSeq.foldAsync functions

[abelbraaksma]

This adds the following two functions and a few tests, which should greatly simplify accumulating a value throughout an IAsyncEnumerable.

I should probably emphasize already that I'm not planning to have a TaskSeq.foldBack and TaskSeq.foldBackAsync, as it would require walking the full async sequence, caching it, and then reverse-accumulating, which defeats the purpose of a delay-executed asynchronous sequence. If you have that use-case, you can use TaskSeq.toArrayAsync >> Task.map (Array.foldBack yourFolderFunction yourState).

module TaskSeq =
    /// Applies a function to each element of the task sequence, threading an accumulator argument through the computation.
    val fold: folder: ('State -> 'T -> 'State) -> state: 'State -> taskSeq: IAsyncEnumerable<'T> -> Task<'State>

    /// Applies an async function to each element of the task sequence, threading an accumulator argument through the computation.
    val foldAsync: folder: ('State -> 'T -> #Task<'State>) -> state: 'State -> taskSeq: IAsyncEnumerable<'T> -> Task<'State>