Generating sql

## Sketch of idea of how to convert tree of expressions to sql.

Main idea is that every child of `Operator` would have `.to_sql` method and we would just simply call them recursively (base case), but for scalar values (strings, numbers) we have to create `ValueExpression` (boundary case) which just returns value that he holds.

We could implement it even without `ValueExpression`, but we would have to check in every `.to_sql` if our children are scalar values or another Operations.


Implemented use cases for `And` and `Equal` operators.