More IntermediateSerializer

This is a follow up to https://github.com/mono/MonoGame/pull/2621.

With this PR i've been able to do some limited deserialization using `IntermediateSerializer`.

In the process of getting this working I've noticed that the design of the `IntermediateSerializer` is very much like that of the `ContentTypeReader`/`ContentTypeWriter` system.  I'm using the setup of that system as additional help to the blog posts and MSDN docs.

We currently only support float, int, and string fields/properties, but now that I have the system basically working it is easy to add serializers for a bunch of additional types.

No support for deserialization of collections/dictionaries/arrays/lists yet.

No serialization (writing) support yet, that will most likely be last.

Note i'm being careful to be thread safe with the design here, so we can enable parallel processing of content in the upcoming weeks.