Compile-time-safe type converters

[cyrillhalter]

Implements a new annotation based type converter interface. The main features are:

• it is ensured at annotation processing time that all Fields that need one have a type converter
• the type converter specifies both the input and the output class
• type converters can be exported in one project and imported in another for reuse
• a new generic Enum converter is added
• kotlin poet was updated