Split functions into front-end and back-end functions

[jarmuszz]

Currently, some user functions call other user functions which is a really bad practice. Some functions should be split into front-end and back-end parts (like _tree and intern_tree) where front-end function handles arguments and error-checking and calls the back-end function/s.

Split like this will not only improve performance but will also fix bugs and and make debugging easier.

Functions to split:

• _goto
• _find
• _source_info
• _unset_info
• And probably others