Implement `max|min`, `maxBy|minBy` and `maxByAsync|minByAsync`

[abelbraaksma]

Part of the push for good coverage of surface area functions, see #208. This implements min, max, minBy, maxBy, minByAsync and maxByAsync. Each of these behave exactly like their Seq counterparts:

• raises on null input
• raises if the input is the empty task sequence
• returns the min or max of the input by using IComparable semantics (i.e.: when 'T: comparison)

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 max: source: TaskSeq<'T> -> Task<'T> when 'T: comparison
static member max: source: TaskSeq<'T> -> Task<'T> when 'T: comparison

static member maxBy: projection: ('T -> 'U) -> source: TaskSeq<'T> -> Task<'T> when 'U: comparison
static member minBy: projection: ('T -> 'U) -> source: TaskSeq<'T> -> Task<'T> when 'U: comparison

static member maxByAsync: projection: ('T -> #Task<'U>) -> source: TaskSeq<'T> -> Task<'T> when 'U: comparison
static member minByAsync: projection: ('T -> #Task<'U>) -> source: TaskSeq<'T> -> Task<'T> when 'U: comparison

TODO list:

• [x] Implement TaskSeq.min
• [x] Implement TaskSeq.max
• [x] Implement TaskSeq.minBy
• [x] Implement TaskSeq.maxBy
• [x] Implement TaskSeq.minByAsync
• [x] Implement TaskSeq.maxByAsync

[abelbraaksma]

Tests I'll do tomorrow.

[abelbraaksma]

This is ready to be merged.