Support for netstandard 2.1

[xivk]

Support netstandard2.1 and all the goodies we can use in this library for that. See also #11

https://github.com/dotnet/standard/blob/master/docs/versions/netstandard2.1.md

[airbreather]

One thing to point out is that legacy .NET Framework 4.8 does not implement .NET Standard 2.1, and per this blog post, there are no plans for future .NET Framework releases.

So if Itinero needs to support legacy .NET Framework targets, then netstandard2.1 should not be the only target framework.

See also the announcement post: https://devblogs.microsoft.com/dotnet/announcing-net-standard-2-1

[xivk]

I dropping support for .NET Framework is ok mid-term in favor of .NET core.