Add the ablitity to handle the parse tree directly

It should be possible to handle the parse tree directly too, not only having Booleano evaluating the the operations internally.

This way, developers could take the parse tree and transform it into something more useful for them. For example, they could take the parse tree as an operation (see package booleano.operations) and transform it into a WHERE clause in a SQL query.

To support the parse tree in the form of a Booleano operation which isn't going to be evaluated, there must be created "placeholder structures" for variables and functions, because such structures wouldn't be available in such situations.