non symmetricity between `fun` and `function` for tuple forces usage of parens

those are allegedly similar semantically:
```fsharp
fun a -> //...
function a -> //...
```

but for tuple, it doesn't work
```fsharp
fun a,b -> // error FS0010: Unexpected symbol ',' in lambda expression. Expected '->' or other token.
function a,b -> //...
```

I wonder if this can be fixed without ambiguity, because it would be an incomplete lambda in current version of the language and it would reestablish fuller symmetry with `function` construct.

**The existing way of approaching this problem in F# is** to use `function` or wrap with parens.

Maybe a code fix for putting the parens, or turning into `function` would make sense if we can't make this suggestion work / it is rejected on some grounds.

## Pros and Cons

**The advantages of making this adjustment to F# are** semantically more consistent.

**The disadvantages of making this adjustment to F# are** may prevent other constructs in the future.

## Extra information

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

**Related suggestions:** (put links to related suggestions here)

## Affidavit

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.
* [x] This is not something which has obviously "already been decided" in previous versions of F#. I
* [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
* [ ] 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.

The issue names no implementation files or tests. Start by reproducing the `fun a,b ->` and `function a,b ->` examples in F# and review the language-design discussion in the seven comments. Done requires an accepted resolution for the ambiguity, along with corresponding compiler behavior and tests.

Written by the indexing model from the issue text.