Add a Unity-compatible Boxed.Mapping.Unity package

[Razenpok]

This PR adds a new Boxed.Mapping.Unity package that fixes Unity incompatibilities described in #592

Implementation notes:

• I've decided to not introduce a new UNITY define symbol. Instead, I've made a separate copy of the Factory.cs file and added a <Compile Remove> tag to the .csprojs. I think this is a more readable and maintainable approach.
• I've added [MethodImpl(MethodImplOptions.AggressiveInlining)] attribute to the Factory<T>.CreateInstance() method. This will hint Unity to try and inline the method.
• I don't know all the details of the build pipeline, so you might need to do some additional actions for the package to appear on NuGet

Closes #592

[RehanSaeed]

Build is failing. Haven't had time to take a look yet.

[Razenpok]

Okay, thanks for update

[Razenpok]

@RehanSaeed any updates on this one?

[RehanSaeed]

Apologies, I'm having difficulty finding the time. Not had a chance to see what is wrong with the build.