Generic programming with extensible transformations

Yet another library for generic programming. Provides syntax extensions both for camlp5 and PPX which allow decoration of type declarations with following compile-time code generation. Provides the way for creating plugins (compiled separately from the library) for enchancing supported type transformations.

Strongly reminds the visitors library from François Pottier. During desing of a library of these kind there many possible design decision and in many cases we decided to implement the decision opposite to the one used in visitors.