Cross target netstandard2.0

[iamcarbon]

This PR adds netstandard2.0 to the target frameworks. This simplifies the dependency tree on modern runtimes.

[ob-stripe]

Hi @iamcarbon, thanks for the contribution!

Can you clarify how this helps exactly? I'd've thought that libraries would only need to target the minimum version of .NET Standard that they require. Is that not the case?

[iamcarbon]

Hey there. Targeting NETSTANDARD2.0 directly eliminates the 50+ dependencies that come from .NETSTANDARD 1.x libraries. .NETSTANDARD2.0 reduces this to a single dependency (netstsandard.dll) that is already included in modern runtimes (netfx472, netcoreapp2.0, etc).

The official guidance from MSFT is now to re-baseline at 2.0x or multi-target:
https://twitter.com/davidfowl/status/1008453902058917888

[ob-stripe]

Cool, thanks @iamcarbon. Going to pull this in.

[ob-stripe]

Released as 19.9.2.

[ob-stripe]

Just for future reference in case someone stumbles upon this: https://github.com/dotnet/standard/blob/master/docs/versions.md#how-do-i-know-which-net-standard-version-i-should-target

• You should target the lowest version you can get away with.
• If you target .NET Standard 1.x, add .NET Standard 2.0 as another target.