Allow [<StructuralEquality>] and [<StructuralComparison>] for simple class types

**I propose we** allow `[]` and `[]` on simple class types

**The existing way of approaching this problem in F# is** to write the equality / comparison members oneself.

## Pros and Cons

**The advantages of making this adjustment to F# are** to help reduce the dissonance between classes and records (without requiring us to add more and more class-like features to records).

**The disadvantages of making this adjustment to F# are** just the work involved in implementing such a feature.

## Extra information

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

Related suggestions: #553

## Affidavit (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