Upgrade definition file to TypeScript v2.2.+

[gcanti]

This will be a major refactoring (and likely a breaking change for TypeScript users). There are some techniques developed in io-ts that can be ported to tcomb, in particular the usage of indexed and mapped types

[jaen]

Incidentally, I have some more accurate typings for struct that I needed lying around, not sure if they will be helpful since they are pretty rudimentary, but no harm done linking to them: https://github.com/jaen/tcomb/commit/943e8eb3aec290ac0eec8703ae5add115f085437

[DanielSWolf]

What I really like about io-ts is the fact that a runtime type definition also serves to define the compile-time TypeScript type. That is, I only have to define a given type once. tcomb, on the other hand, allows me to do runtime type validation, but I also have to define a (redundant) TypeScript type.

Are you planning to implement something similar in this issue?