rewrote expression parser with a new algorithm to limit the depth of nested calls

rewrote expression parser with a new algorithm to limit the depth of nested calls

This change has no impact for the end user, but many internal benefits:

* less nested calls (was the primary reason for the change as xdebug limits the depth of nested calls);
* less code;
* code is much cleaner (I have removed all the parsing "hacks");
* faster;
* more flexible (we will now be able to expose an API to add new operators);
* easier to maintain.