Refactoring:
- create an empty_statement_t and use that instead of a NULL pointer in places
  like if, loop bodys, statements after labels
- eliminate target_architecture.h and replace with stuff in lang_features.h
- redo storage classes: so we can separate real from declared storage class

Lexer:
- Add preprocessor code
- proper handling of different file encodings, parsing non-ascii strings
- We could save some space by holding source positions in a separate (somehow
  compressed table) and only storing pointers to it on the AST.

Parser:
- disallow storage class specifiers in sturct/union members
- the expect macros abort functions directly. This leads to some functions
  not resetting the current context properly (parse_for); expect in expressions
  suddenly return NULL which triggers asserts
- label: declaration; is no valid C99 but we parse it anyway
- add constant folding code (optional, ast2firm already does it)
- 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
- Add columns to source positions
- SourcePositions could be stored selectively on expressions that really need
  them.
- check semantic for functions declared/defined in global scope and declared
  again in a local scope
- print initialisation type_path for initializer errors

ast2firm:
- handle non-constant initializers
- output source file positions for panics...
- handle bitfield members with 0 correctly (standard says they finish the
  current unit)

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