Re-work type system section to introduce a consistent language

[VasiliyLysokobylko]

The current RAML 1.0 RC1 version of type system, does not have a consistent way for describing types. All what we have is the Types Overview section which introduces common definitions, but is does not actually explaining anything. Therefore, it's hard to read and understand all sub sequent sections.

RC2 should introduce a framework which allows a consistent understanding of what types are and how to apply complex structures/restrictions & algorithms for example for multiple inheritance.

We hope that the new section will help people to better understand types, how to use them, and how to implement something on top of it.

This change is more content rather than any syntactical or behavioural change!

[VasiliyLysokobylko]

https://github.com/raml-org/raml-spec/issues/305

[petrochenko-pavel-a]

This is not related to parser