LegoStream and implementations 100% match (#77)

* MxFile and implementations 100% match

* Add MxFile and it's implementations, MxSystemFile and MxMemoryFile.
  The names are chosen by me, we don't know their original naming.

* These expose a Read/Write/Seek/Tell interface for reading and writing
  data, either from a file on disk or memory buffer.

* 100% match all functions.

* Change name to LegoStream

* Use p_ convention

* Assert size

CMakeLists.txt

@@ -80,6 +80,7 @@ add_library(lego1 SHARED
   LEGO1/legoroi.cpp
   LEGO1/legosoundmanager.cpp
   LEGO1/legostate.cpp
+  LEGO1/legostream.cpp
   LEGO1/legotexturepresenter.cpp
   LEGO1/legoutil.cpp
   LEGO1/legovideomanager.cpp