Implement TaskSeq.forall and forallAsync

[abelbraaksma]

Part of the push for good coverage of surface area functions, see #208 for an overview. This implements TaskSeq.forall, TaskSeq.forallAsync.

Each of these behave exactly like their Seq counterparts:

• raises on null input

As before, the xml doc blibs have been taken from seq.fs and modified a bit for readability and applicability to TaskSeq.

The signatures are as follows:

static member forall: predicate: ('T -> bool) -> source: TaskSeq<'T> -> Task<bool>
static member forallAsync: predicate: ('T -> #Task<bool>) -> source: TaskSeq<'T> -> Task<bool>

TODO list:

• implement TaskSeq.forall
• Implement TaskSeq.forallAsync
• unit tests

[abelbraaksma]

Unit tests added, rebased, should be good to go now. This was the last PR related to the list in #208.