(compiler pl0): add identity / number lex support.

Also lex for keywords.
(compiler pl0): add comment support.
(compiler pl0): add lexer testcases.
(compiler pl0): add write / read keyword.

And remove token value for keywords.
(compiler: pl0): add a complicated example.

Also close src stream after testing.
(compiler pl0): reset lines / cols when setting stream.
(compiler pl0): add period token.
(compiler pl0): fix ident pattern.

It should be: [a-zA-Z][a-zA-Z0-9]{0,}.
(compiler pl0): add ident length check.

Also refactor test script.