Started to remove curiously recurring template parameter.
Killed Widelands::FileRead and Widelands::StreamRead.
Distributed the remaining functions in widelands_streamread.cc logically.
Removed mmap()ed reading. It is not really useful for the kind of small files we are reading.
Take std::string instead of char*.
Removed curriously recurring template pattern from StreamWrite.
- Refactors io/ and classes that use it to not use the curiously reoccurring
  template pattern anymore.
- Removes mmap() usage for reading. It does not benefit us with our many small
  files and makes the code more complicated.
- Removes widelands_streamread* and widelands_fileread* classes and replaced
  their functionality through free functions.