Add netstandard2.0 target to FSharp.Core project and nuget package

[jack-pappas]

I noticed recently the FSharp.Core package on Nuget claims to support net45, netstandard1.6, and netstandard2.0. However, after downloading and inspecting the package, I found it only has net45 and netstandard1.6 binaries but no netstandard2.0 binaries.

This isn't a huge deal -- you can use the netstandard1.6 version of FSharp.Core when e.g. building a library targeting netstandard2.0. It is a bit annoying though, because referencing the netstandard1.6 version of FSharp.Core causes some additional packages (which aren't needed for netstandard2.0) to be pulled into a build and referenced by any netstandard2.0 libraries built with F#.

I've simply added the additional netstandard2.0 target framework to the FSharp.Core project + .nuspec files here, which means we'll actually have a netstandard2.0 version of FSharp.Core in the Nuget package, as advertised.

This change can stand on it's own*, but at present will still be using some conditional-compilation workarounds needed for netstandard1.6. I've submitted a separate, also-standalone PR in #6955 to address that.

• = If Refactor FSharp.Profiles.props for finer-grained constant definitions #6955 is not going to be merged, this PR will need one additional change made to undefine the FX_NO_REFLECTION_MODULE_HANDLES constant for the netstandard2.0 build, by modifying either quotations.fs or FSharp.Profiles.props.

[matthid]

please see #3454 (comment)
In the past reasoning was that there are hard restrictions on package size of FSharp.Core to get bundled with dotnet sdk... This is the reason it looks this way - which apparently is a supported scenario (at least no issue was reported for years)

[cartermp]

@jack-pappas see here: #6814

We will be .NET Standard 2.0 for the F# 4.7 release later this year.

Because .NET Core 1.x support ends on June 27th, we still need to support a netstandard1.6 component as it was designed to work with that .NET Core version.

[KevinRansom]

@jack-pappas

this is already taken care of the branch fsharp47, that will make it's way back into master later in the year. Until then master needs to produce fsharp.core for netstandard1.6

Kevin