follow_statement -> eval_statement
follow_call -> eval_call
use expression_list instead of commands or call_list
expression_list instead of commands in more places
pep8 stylings
follow_call_path -> eval_call_path
follow_paths -> follow_path, follow_path -> _follow_path
improve follow_path params
use execute method instead of Execution creators
basic refactoring, of function executions super() is not working yet
add a stdlib module to make it easy to write functions in pure python (instead of evaluating them)