Added missing interface condition() to IExpr

Updated Expr class' constructors to use baseExpr fields for all
of this' fields wherever applicable
Resolved a FIXME in Expr.value() method by removing the controversial
piece of code
BasicSolver now has code to handle expressions

This is in no way a complete solution and is almost guaranteed to
have bugs. However, it is a decent platform to build up from.
Added sample files for simple dependency in definitions