Updated an example file

Regular expressions now prefixed with r, error statements now use a more traditional string.

These changes should ease lexical analysis without really negatively impacting the language
Made keywords and identifiers matched by the same code

A TextToken (sort of a pseudo-token) matches identifier and keyword like text and then decides whether to produce a KeywordToken or IdentifierToken by seeing if the text matches a keyword
Split lexer tests up into a sub-package