Lexer:
- Add preprocessor code
- proper handling of different file encodings, parsing non-ascii strings

Parser:
- the expect macros abort functions directly. This leads to some functions
  not resetting the current context properly (parse_for)
- label: declaration; is no valid C99 but we parse it anyway
- designator support for initializers
- add constant folding code
- Refactor code, so code to handle number values and strings is
  an own module and replacable
- Support some attributes. noreturn, unused, printf, scanf, packed would be
  interesting candidates

ast2firm:
- output source file positions for panics...
- output variable names for uninitialized variable warnings

Missing Warnings:
* dead assignments (int x = 5; x = bla(); -> dead assignment at x = 5;)
* unused label
*