IBufferWriter is part of the standard runtime and since we're
not using the property introduced by our interface ICountableBuffer
(which extends IBufferWriter)there is no reason to keep that
dependency on the public interface.

* Simplified INatsSerializer a little more. (Serialize method's return
  value wasn't used)

* Removed the default JSON serializer which was serializing any object
  using reflection. Reflection isn't possible with native AOT. This
  also makes serialization more explicit.

* Added a UTF8 primitives serializer in the default serializer chain
  to cover simple use cases like sending strings or ints.

* Added JSON serializer contexts for JS, Obj and Services models.

Co-authored-by: Jasper <jasper-d@users.noreply.github.com>

Co-authored-by: Jasper <jasper-d@users.noreply.github.com>

Co-authored-by: Jasper <jasper-d@users.noreply.github.com>

Also removed method inlining on getters (based on Jasper's advice:
Presumably JIT should always inline simple field accesses and the
other getters look like they are likely inlined without
AggressiveInlining when on a hot path.)

Thank you @jasper-d

Note that chaining serializers is implemented by convention and doesn't need
to be enforced by the INatsSerializer interface since the next serializer
would not be exposed to external users of the interface.

All primitives supported by Utf8Parser and Utf8Formatter
are implemented.