matroska: add new parsing code

matroska: add new parsing code

Add a new EBML parser implementation that should allow significant
improvements to the Matroska demuxer. The new parsing code is not
actually used yet by the demuxer. The only changes to existing code in
this commit are to generate the MATROSKA_ID_* / EBML_ID_* macro
definitions from the new implementation and to rename some of them
(the new implementation uses names matching the official Matroska spec).

The main parser implementation is added in ebml.c. There are two new
generated files, ebml_defs.c and ebml_types.h, that contain
definitions of EBML elements. Those are generated by the new script
TOOLS/matroska.py. There's a new Makefile target "generated_ebml" that
run the script to refresh the content of the generated files.