Generic struct type whose fields are all unmanaged types is unmanaged

# Generic struct type whose fields are all unmanaged types is unmanaged

> A non-generic struct type whose fields are all unmanaged types is unmanaged.

into

> A generic struct type whose fields are all unmanaged types is unmanaged.

as

> The unmanaged constraint on generic type T enforces the type to be an 'unmanaged' struct that does not recursively contain reference type fields.

as for cases
```
type Cell<'T when 'T: unmanaged> =
struct
val element: 'T
val next: nativeptr>
end

type UnmanagedCell<'T when 'T: unmanaged> =
struct
end

[]
let main argv =
let fine = UnmanagedCell()
let FS0001 = UnmanagedCell>()
0
```

The existing way of approaching this problem in F# is prevent compilation with error

> error FS0001: A generic construct requires that the type 'X' is an unmanaged type

I have asked the question on [SO](https://stackoverflow.com/questions/52228109/a-generic-construct-requires-that-the-type-cellt-is-an-unmanaged-type) and found that it is F# limitation, not CLR.

## Pros and Cons

See `Related suggestions`

## Extra information

Related suggestions:
https://github.com/dotnet/csharplang/issues/1504
https://github.com/dotnet/csharplang/issues/1649

## 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
* [ ] I or my company would be willing to help implement and/or test this

No repository files, tests, or entry points are named. Start with the generic struct examples and the related C# language proposals, then determine the required F# language-design and compiler changes. Done means the proposed unmanaged generic structs compile and the invalid case receives the appropriate constraint diagnostic.

Written by the indexing model from the issue text.