Add parse_statement() member
Add make_snapshot() and rewind()

Add make_snapshot() member to save snapshot of current_.
Add rewind() member to restore snapshot of current_.
Use make_snapshot() and rewind()
Add parse_expression() member

Add also parse_assign_expression() member declaration.
Add parse_assign_expression() member

Add also parse_or_expression() member declaration.
Add parse_or_expression() member

Add also parse_and_expression() member declaration.
Add parse_and_expression() member

Add also parse_comparative_expression() member declaration.
Add parse_comparative_expression() member

Add also parse_additive_expression() member declaration.
Add parse_additive_expression() member

Add also parse_multiplicative_expression() member declaration.
Add parse_multiplicative_expression() member

Add also parse_unary_expression() member declaration.
Add parse_unary_expression() member

Add also parse_postfix_expression() member declaration.
Add parse_postfix_expression() member

Add also following member declarations.
* parse_parameter_list()
* parse_primary_expression()
Add parse_parameter_list() member
Add parse_primary_expression() member