Upgrade CSF.Core library to v3.2.0
WIP #10 - Add an integration test that highlights this issue.

This commit does no work that will fix the issue, rather it just introduces a
failing test to demonstrate the problem.
Resolve #11 - Add factory type for creation of key/value serializer instances

This factory type provides multiple ways in which serializers may be
constructed, and is backed by an interface (for mocking and stubbing).

* Add a (currently unused) concept of SerializerOptions, which may be used
  alongside the factory.

* Add new (more tersely named) interfaces for the various serializer types.

* Add new interface for a value-type serializer.  This won't be super-useful,
  since it can really only have either a single 'composite' or 'simple' mapping
  but it is there for completeness.  It still needs to be implemented though.