Increase surface area: TaskSeq.length, lengthBy and lengthByAsync

[abelbraaksma]

This PR adds the following to the surface area, as part of #38:

/// <summary>
/// Returns the length of the sequence. This operation requires the whole sequence to be evaluated and
/// should not be used on potentially infinite sequences.
/// </summary>
val length: source: taskSeq<'T> -> Task<int>

/// <summary>
/// Returns the length of the sequence of all items for which the <paramref name="predicate" /> returns true.
/// This operation requires the whole sequence to be evaluated and should not be used on potentially infinite sequences.
/// </summary>
val lengthBy: predicate: ('T -> bool) -> source: taskSeq<'T> -> Task<int>

/// <summary>
/// Returns the length of the sequence of all items for which the <paramref name="predicate" /> returns true.
/// This operation requires the whole sequence to be evaluated and should not be used on potentially infinite sequences.
/// If <paramref name="predicate" /> does not need to be asynchronous, consider using <see cref="TaskSeq.lengthBy" />.
/// </summary>
val lengthByAsync: predicate: ('T -> #Task<bool>) -> source: taskSeq<'T> -> Task<int>

[abelbraaksma]

Let's get this in. It already contains more than just this change.