Add MergeSources and/or BindN to AsyncBuilder

# Add `MergeSources` and/or `BindN` to `AsyncBuilder`

I propose we add `MergeSources` and/or `BindN` to `AsyncBuilder`.

The existing way of approaching this problem in F# is to add it yourself.

```F#
module Async =
let map f x =
async.Bind (x, f >> async.Return)

let zip (x : Async<'a>) (y : Async<'b>) : Async<'a * 'b> =
[| map box x; map box y |]
|> Async.Parallel
|> map (fun xy -> unbox xy.[0], unbox xy.[1])

type AsyncBuilder with
member _.MergeSources (x, y) = Async.zip x y
```

## Pros and Cons

The advantage of making this adjustment to F# is that
```F#
async {
let! _ = Async.Sleep 10_000
and! _ = Async.Sleep 10_000
return () }
```

runs in half the time of

```F#
async {
let! _ = Async.Sleep 10_000
let! _ = Async.Sleep 10_000
return () }
```
and making it official gives us a more compelling example of applicative CE usage, in my opinion, than the one in the release announcement.

The disadvantage of making this adjustment to F# is that some code will be added, especially if we think it's worth doing better than the simple implementation above (perhaps for an optimized implementation of `map`/`BindReturn`).

## Extra information

Estimated cost (XS, S, M, L, XL, XXL): S

Related suggestions:
https://github.com/fsharp/fslang-suggestions/issues/859

## Affidavit (please submit!)

Please tick this by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [stackoverflow](http://stackoverflow.com)) and I have searched stackoverflow for discussions of this issue
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [x] I or my company would be willing to help implement and/or test this

## For Readers

If you would like to see this issue implemented, please click the :+1: emoji on this issue. These counts are used to generally order the suggestions by engagement.