New module for config evaluation. Basic testing.

New module for config evaluation. Basic testing.

Implemented:
* Configuration tree,
* Variables,
* Variable-setting operations ('SET' and 'APPEND')
* Equality conditions (WIP, not tested yet),
* Substitution expressions with text and variables,
* Pretty-printing,
* Caching for variables and conditions with invalidation along reverse-dependencies,
* Add and remove operations from variables with correct reverse-dependency handling (add and remove)

TODO: smarter conditions, variable lookup (and maybe transparent creation),
possibly cleanup of unused variables and conditions

(tests full coverage except conditions)