Rename statements

Change the names of the statements:
- `IF` -> `if`
- `ELSEIF` -> `elseif`
- `ELSE` -> `else`
- `TRY` -> `try`
- `CATCH` -> `catch`
- `WHILE` -> `while`
- `FOR` -> `for`
- `PARFOR` -> `parfor`
- `ASYNC` -> `async`
- `RETURN` -> `return`
- `POP` -> `pop`
- `BREAK` -> `break`
- `CONTINUE` -> `continue`
- `GOTOPOINT` -> `gotopoint`
- `GOTO` -> `goto`