Improve expression visitor

[vitaut]

Currently, ExprVisitor handles NumericExpr and LogicalExpr hierarchies separately. This was done because the result types of expression conversion for numeric and logical expressions are normally different (this is true for gecode and ilogcp). Need to add support for Expr, including string expressions. Among other things this will be useful for completing structural comparison of expressions.

Possible ways to implement:

• Add another Visit(Expr) overload and dispatch on all expression kinds. Drawback: duplicates numeric and logical expression dispatch.
• Replace Visit(NumericExpr) and Visit(LogicalExpr) with Visit(Expr). Since these methods use jump tables, it shouldn't affect performance, but need to handle different return types for logical and numeric expressions somehow.

[vitaut]

Implemented the second option (one dispatch for all expression types) in fe42f07. Now it's possible to handle arbitrary target expression hierarchies. Need to implement string expressions and update solver drivers.

[vitaut]

Finished refactoring and added support for StringLiteral and SymbolicIf in 041583b.