Add `TaskSeq.concat` overloads for seq, list, array, resizearray

[abelbraaksma]

Part of the push for good coverage of surface area functions, see #208 for an overview. This implements TaskSeq.concat overloads for seq, list, array and ResizeArray. Reasoning behind specific overloads is that multiple overloads with flex types is not possible: https://github.com/dotnet/fsharp/issues/4943.

Each of these behave exactly like their Seq counterparts:

• raises on null source
• raises normal NullReferenceException when a nested sequence returns null

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 concat: sources: TaskSeq<#TaskSeq<'T>> -> TaskSeq<'T> // already there
static member concat: sources: TaskSeq<'T seq> -> TaskSeq<'T>
static member concat: sources: TaskSeq<'T[]> -> TaskSeq<'T>
static member concat: sources: TaskSeq<'T list> -> TaskSeq<'T>
static member concat: sources: TaskSeq<ResizeArray<'T>> -> TaskSeq<'T>

TODO list:

• [x] implement TaskSeq.concat for seq
• [x] implement TaskSeq.concat for array
• [x] implement TaskSeq.concat for list
• [x] implement TaskSeq.concat for ResizeArray