Add argmax,argmin equivalents to FSharp.Core

NumPy has `argmax` and `argmin` to return the index of the maximum and minimum values in a collection.

I propose we add something similar to F#, either with these names of `indexOfMax` and `indexOfMin`

The existing way of approaching this problem in F# is a little painful. You have to find the max and then find the specific element with that max.

```fsharp
module List =
let indexOfMax xs =
let mx = List.max xs
xs |> List.findIndex (fun v -> v = mx)
```
Not that bad but seems reasonable to have this in the core library, optimized appropriately and inlined to avoid the generic comparisons.

The functions occurs in basic machine learning samples like https://www.tensorflow.org/tutorials/keras/basic_classification

## Pros and Cons

The advantages of making this adjustment to F# are simpler code in some scenarios

The disadvantages of making this adjustment to F# are added functions in FSharp.Core.

## Open questions

* [ ] are the names what they need to be? Would `maxByIndex` by better than `indexOfMax`? Should it be `findIndexOfMax` to fit with `findIndex`. Other suggestions?
* [ ] should there by any similar functions, e.g. `indexOfMaxBy` seems reasonable.

## Extra information

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

## 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