Generic attributes

# Generic attributes

[Champion "Allow Generic Attributes"](https://github.com/dotnet/csharplang/issues/124) is currently in the C# team's working set.

Since [CIL already supported this for a decade](https://stackoverflow.com/a/294259/5429648), if F# were to implement it, we won't have to wait for C# to have this.

## Pros and Cons

The advantages of making this adjustment to F# are
1. Enforcing generic restrictions at compile time which `typeof` cannot do
2. Type-safe attribute members without type-unsafe boxing conversions with reflection
3. Generics is faster than reflection

The disadvantages of making this adjustment to F# are (none)

## Extra information

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

Related suggestions: (put links to related suggestions here)

## 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

## 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 files, tests, or F# entry point. Start by reading the linked C# champion proposal and the referenced CIL generic-attribute support, then determine the required F# language design, implementation scope, and tests needed for generic attributes to be complete.

Written by the indexing model from the issue text.