Class as method name

`class` is now treated as an identifier unless its next token is
constant.
This allow as to use ...

Support comment

Method call without specifying self

For example: 'self.bar;' can be written like 'bar()'.
However, we must put parenthese after meth...

Remove let keyword from variable assignment.

Statement can avoid '()' if there's no parameters needed, for example:
```
class Foo {
def bar...

* Token now contains line number.

* Add line number into parsing error message.

Enable initialize instance with defined 'initialize' method

Inheritance

Instance variable

first instance method call!!!!!!.