Merge immutable collections formatters into main library

[AArnott]

This removes the need for a separate assembly and package to support immutable collection types being serialized/deserialized. It also removes the need to manually link in the immutable collection resolver as it is now included in the standard resolver.

This requires that we ship at least one more version of the MessagePack.ImmutableCollection package in order to deliver the type forwarders. After that, we could delete the project from the repo and never ship it again. The function of the type forwarders is to that if someone compiled an assembly that references MessagePack.ImmutableCollection.dll, the CLR will demand that that dll be present at runtime with those types. The type forwarders allow the CLR to recognize that those types moved from one DLL to another and keep functioning. The DLL is only necessary at runtime when other dll's were compiled against it -- otherwise it can be omitted from an application.

Closes #606

[neuecc]

ImmutableCollection should not add under Unity.

[AArnott]

Thanks for reviewing, @neuecc. Are you saying the formatters should not be included in the Unity version? If we exclude it, how do you see Unity customers adding immutable collection support if they want it? Or are immutable collections broken under unity anyways?

[neuecc]

@AArnott
We have two choices.

• Include System.Collections.Immutable.dll in Unity's package(your PR have xcopy, so currently choosed this)
• Exclude Formatter/Resolver of ImmutableCollection

Until now, there was no way to add a formatter for the ImmutableCollection (other than by copy-paste from the source code)

As far as I'm concerned, I think it's fine to exclude it completely.
First of all, as I've been repeating all along, it's very important to minimize dependencies.
Especially in Unity, where there is no way to resolve assembly versions, adding unnecessary dlls should be avoided.
And Immutable Collections are completely unattractive and undemanding in Unity (lots of allocations are not preferred in games).
If you really want to add it, you can cover it with copy and paste (you can add the .unitypackage independently, but there's little demand for it)

[AArnott]

OK. Thanks for the clarification. I'll omit these from the unity package.

[neuecc]

Thanks, okay to merge.