- interface is analog of interface{} in Go.
- error is analog of error in Go.

For now the interfaces are implemented via classic C++ scheme via
inheritance and additional vtable field in pointed object.

In the future we might want to consider reworking that to Go-style
interfaces without predeclaring which interfaces a class implements.

Top-level documentation is TODO.

Interfaces will be soon needed to describe Context interface in C
context package.