Netstandard2

[KevinRansom]

Original PR: #6771

FSharp.Core is updated to target netstandard2.0 as the minimum required platform.
The FSharp.Core nuget package version 4.7.0, will contain net45 and netstandard2.0 dlls
with the version number 4.7.0.0

This PR enables the FSharpFunc API's that use the System.Converter class.
Apparently the API Surface area tests have not been running, so I have re-enabled them, added back in the APIs that were missing. Some have moved around the baseline.
master will not be updated to a netstandard2.0 until after VS2019.2 has RTM'd

[dsyme]

This makes me so happy :)

[dsyme]

I'm struggling to understand why this went into release\fsharp47 instead of master or release\dev16.2 or ....

[cartermp]

@dsyme Because netstandard1.6 is for code sharing with the .NET Core 1.x release set, and .NET Core 1.x is still in LTS until this end of June 27th, we decided to keep theofficially supported release with this as the target until it is deprecated.

Since we need code flow from master -> release/dev16.2 and into release/dev16.2 and don't want to pull in this as a takeback while it's still in LTS, we're holding this off in release/fsharp47, which will merge into the release/dev16.3 release (which is also the .NET Core 3.0 release).