Allow attributes for discriminated union case fields

# Allow attributes for discriminated union case fields

I propose we allow the specification of attributes on case fields of discriminated unions.

Here's an example:

```
type A =
| ACase of [] SomeField
| BCase [] SomeFieldName: SomeType
```

The existing way of approaching this problem in F# is none, if we want case fields to be decorated

## Pros and Cons

The advantages of making this adjustment to F# are
* It opens up more possibilities of what one can do with reflection using F# types
* Since the union case fields are simply .NET properties, it helps with other libraries that expect attributes

The disadvantages of making this adjustment to F# are none

## Extra information

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

## Affidavit (please submit!)

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 or my company would be willing to help implement and/or test this