.NET 3.5 via MonoMod.Backports

[Washi1337]

• Adds .NET 3.5 build targets for all main packages.
• Adds MonoMod.Backports as a conditional package reference for .NET 3.5 build targets.
• Adds a few extra runtime-abstracting shims to the AsmResolver common package.

Closes #536

[nike4613]

Backports has a few of these shims already, on *Ex suffixed types in the same namespace as the relevant type. (Though I see you're limiting the Backports package ref to net35, so they may not actually be terribly useful...)

[Washi1337]

Though I see you're limiting the Backports package ref to net35

Yes, I intend to only include MonoMod.Backports when absolutely necessary (i.e., only for the .NET 3.5 target). Using the existing shims for Array and String would therefore not work out.