Support deserialization of custom primitive types

[ftomassetti]

Based on the discussion we had during the meeting on the 25th of May, my understanding is that the different implementation of the libraries can provide custom deserializers for primitive types, as this is not something defined by the specification (neither required or forbidden).

So I thought I could put together an initial implementation to start the conversation.

In this PR we:

• Permit to define custom primitive type deserializers
• We convert the builtin primitive types to use those primitive type deserializers. That also means that someone could, potentially, override the default way in which primitive types are serialized
• Add a simple test to check the mechanism

If the general approach works I could also add support for primitive type serializer.

I am no TS or functional code expert, so I thought it made sense to open a PR early to check if I am going in a direction that makes sense for the library. Feedback is very well appreciated :)

Fix #156

[ftomassetti]

@dslmeinte thank you for the review, I should have applied the suggested corrections

[ftomassetti]

Sorry: some more suggested refactorings...

Thank you, I think they are helping making the solution better

[ftomassetti]

@dslmeinte sorry for all the troubles. At some point WebStorm did not push the commits and I did not noticed that