[FS-1045] Add FuncConvert.FromFunc, FuncConvert.FromAction APIs

[dsyme]

Continuation of #3013 (comment)

RFC https://github.com/fsharp/fslang-design/blob/master/RFCs/FS-1045-func-to-fsharpfunc-overloads.md

Addresses #1847 - ToFSharpFunc(Func) missing in .NET Core.

Adds

type FuncConvert = 
    static member  inline ToFSharpFunc       : func:Func<'T,'U>          -> ('T -> 'U)
    static member  inline ToFSharpFunc       : func:Func<'T1,'T2,'U>          -> ('T1 * 'T2 -> 'U)
    static member  inline ToFSharpFunc       : func:Func<'T1,'T2,'T3,'U>          -> ('T1 * 'T2 * 'T3 -> 'U)
    static member  inline ToFSharpFunc       : func:Func<'T1,'T2,'T3,'T4,'U>          -> ('T1 * 'T2 * 'T3 * 'T4 -> 'U)
    static member  inline ToFSharpFunc       : func:Func<'T1,'T2,'T3,'T4,'T5,'U>          -> ('T1 * 'T2 * 'T3 * 'T4 * 'T5 -> 'U)
    static member  inline ToFSharpFunc       : func:Func<'T1,'T2,'T3,'T4,'T5,'T6,'U>          -> ('T1 * 'T2 * 'T3 * 'T4 * 'T5 * 'T6 -> 'U)
    static member  inline ToFSharpFunc       : func:Func<'T1,'T2,'T3,'T4,'T5,'T6,'T7,'U>          -> ('T1 * 'T2 * 'T3 * 'T4 * 'T5 * 'T6 * 'T7 -> 'U)

There was no good way to create an FSharpFunc object from C# on .NET core with a method call.

[dsyme]

I have updated the RFC linked above to cover the updated changes in this PR.

E

[dsyme]

@brettfo @KevinRansom Should updates to FSharp.Core target dev15.8 or master? Thanks

[brettfo]

@dsyme I would say master. We're currently flowing all changes from master into dev15.8 (this will be disabled as we try to stabilize 15.8 at the divisional level), but we're not flowing anything from dev15.8 to master.

[KevinRansom]

Targeting dev15.8

[mdinger]

Not to say this is a bad approach but I've found this usage of invoke leads me to deadlocks when working with async f# with no obvious alternative. Shouldn't there be async friendly alternatives too?