introduce guaranteed backward compatibility

[hanatos]

pre-1.0, vkdt does not give compatibility guarantees, the meaning of parameters might change with every release. for 1.0, we want to guarantee same results (modulo bugfixes) for same parameters.

a good solution is probably:

• assign versions to modules in the .cfg files
• if breaking changes occur, internally convert the version of the module with some upgrade path
• define a set of core modules that guarantee stable interface, mark other modules as unstable