Parser: fixed building the string value for StringExpr.
lexer: big change: removed 'next' and 'prev' from Token.

* The individual tokens are no longer linked with each other.
  This makes scanning much faster and reduces memory consumption,
  but it comes at the cost of flexibility (the tokens array
  must not change during the parsing phase.)
* The Lexer always needs to fully scan the text,
  before the Parser can start parsing.