Allow returning `int` from active patterns that only classing the input

**I propose we** allow returning `int` from active patterns that only classing the input, instead of `Choice`, for better performance

Sample:
```fsharp
// Marking the return type as `int`
// only allow returning the cases defined by the name of the active pattern, no explicit numbers like 1, 2, 3, ...
let (|XmlDoc|SingleLine|Else|) (line: string): int =
if line.StartsWith("///") then XmlDoc
elif line.StartsWith("//") then SingleLine
else Else

// Marking the return type of (|A|B|C|) as `int`
let f ((|A|B|C|) : _ -> int) =
match "some thing" with
| A -> "A"
| B -> "B"
| C -> "C"
```

For a n-cases active pattern, its `int` representation will be:
- case 1: 1
- case 2: 2
...
- case (n - 1): (n - 1)
- case n: exclude above

**The existing way of approaching this problem in F# is** returning `Choice` from those active patterns.

## Pros and Cons

**The advantages of making this adjustment to F# are**
- Fewer memory allocations and better performance

**The disadvantages of making this adjustment to F# are** ...
- Adding one more way to do the same thing.

## Extra information

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

**Related suggestions:** (put links to related suggestions here)
#1041
#612

## 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.

No files, tests, or compiler entry points are named in the issue. Start by identifying the active-pattern implementation and existing tests for Choice-based active patterns; done would include support for the proposed int representation with coverage for the documented matching cases and performance rationale.

Written by the indexing model from the issue text.