Add parse_statement() member

Add also following declarations:
* parse_if_statement()
* parse_while_statement()
* parse_for_statement()
* parse_return_statement()
* parse_break_statement()
* parse_continue_statement()
* parse_variable_definition_statement()
* parse_expression_statement()
Add parse_compound_statement() member
Add parse_if_statement() member

Add also following declarations:
* parse_else_statement()
* parse_expression()
Add parse_else_statement() member
Add parse_while_statement() member
Add parse_for_statement() member
Add parse_return_statement() member
Add parse_break_statement() member
Add parse_continue_statement() member
Add parse_variable_definition_statement() member

Add also parse_variable_definition() declaration.
Add parse_variable_definition() member
Add parse_expression_statement() member
Add parse_expression() member

Add also parse_assign_expression() declaration.