Provide predefined deconstructor for F# records

# Provide predefined deconstructor for F# records

I propose we produce deconstructors automatically for F# records.
The new C# 7 language enhancements allow you to produce custom [deconstructors](https://msdn.microsoft.com/en-us/magazine/mt790184.aspx) to coincide with the introduction of the `ValueTuple` type and tuple support in the language.

It seems that it would make sense for F# records to provide a predefined deconstructor for F records either by default or via an attribute like `[]`.

The existing way of approaching this problem in F# is to roll your own deconstructor via a type extension, but records and deconstructors seem to go hand in hand.

## Pros and Cons

The advantages of making this adjustment to F# are that:

- We would have better interop with C# 7 out the box
- We would reduce the introduction of possible boilerplate code

The disadvantages of making this adjustment to F# are:

- This is yet another example of pre-generated code which some people may not appreciate or may consider yet another concept to learn.

## Extra information

- C# currently does not allow deconstructors for single return values, so a record of 1 item would either need to not provide deconstructor or produce a warning or error.

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

Related suggestions: (put links to reated suggestions here)

## Affadavit (must be submitted)

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 would be willing to help implement and/or test this
* [ ] I or my company would be willing to help crowdfund F# Software Foundation members to work on this

No repository files or tests are identified in the issue. Start by reviewing the linked C# 7 deconstructor reference and the existing type-extension approach; clarify the default-versus-attribute design and the behavior for single-field records before implementation.

Written by the indexing model from the issue text.