Add .NET 2.0, 3.5, 4.0, and .NET Standard 1.0 as additional framework targets

[stakx]

There's no reason why this library couldn't support much earlier versions .NET. The necessary reflection bits should be available on most common .NET platform targets.

• .NET 2.0 might require a different public API since its BCL doesn't have the [Extension] custom attribute.
• .NET Standard <2.0 will require some conditional compilation magic because it requires .GetTypeInfo() in some places.

[stakx]

Done in 840d051d2ed5e228435d905778ed737882c6f4da and 836d581c8fec3e0faa0cfeda100382cb0f826541.

Why are there separate targets for net20, net35, and net40? Wouldn't net20 be sufficient?

Perhaps yes, but there are some small differences between these three target platforms:

• net20's base class library is missing the [Extension] attribute class, so it needs to be declared. This redeclared class shouldn't even be in the assembly when targeting a later framework version.

• net35 and net40 target different versions of the CLR, and might thus reference difference reference assemblies.

Why are there separate targets for netstandard10 and netstandard20? Wouldn't netstandard10 be sufficient?

Perhaps, but again, there are some differences:

• Reflection bits in netstandard1.0 are strewn across both Type and TypeInfo, and some members are called differently, while netstandard2.0 is much more like traditional reflection on the .NET Framework.