Warning when comparing sequences

**I propose we issue a warning when comparing sequences.**

As of now the behaviour of equals/comparison on sequences depends on use of constants and/or what the sequence is backed by.
```fsharp
seq { 1 .. 3 } = seq { 1 .. 3 } // false
seq [ 1; 2; 3 ] = seq [ 1; 2; 3 ] // true
```

Another example is `Seq.groupBy` that in some cases will produce unexpected results.
```fsharp
open System

let n = 10

type A = {
I: int
Xs: int seq
}

let someSeq =
Seq.init n (fun i ->
{
I = i
Xs =
if i >= n / 2 then
seq [ 1; 2; 3 ]
else
seq [ 3; 2; 1 ]
})

// 2
let ok =
someSeq
|> Seq.groupBy (fun x -> x.Xs |> Seq.map id |> List.ofSeq)
|> Seq.map (fun (_, xs) -> Seq.length xs)
|> Seq.length

// 10
let unexpected =
someSeq
|> Seq.groupBy (fun x -> x.Xs |> Seq.map id)
|> Seq.map (fun (_, xs) -> Seq.length xs)
|> Seq.length
```

## Pros and Cons

**The advantage of making this adjustment to F# is that it would be more clear that equality between two sequences does not always use structural comparison.**

**A disadvantage of making this adjustment to F# is that the source of the warning may be confusing (as in the case of `Seq.groupBy`.**

## Extra information

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

## Affidavit (please submit!)

Please tick these items 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] This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to [the compiler and tooling repository](https://github.com/dotnet/fsharp)
* [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
* [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

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.

Start by reviewing the sequence equality and Seq.groupBy examples in the issue, then read the discussion to determine which comparisons should trigger a warning and where that warning should originate. Done means the warning behavior and its scope are agreed, with tests covering the contrasting sequence examples.

Written by the indexing model from the issue text.