Compiler: Remove usage of PyDatagram

Usage of PyDatagram was removed, and replaced with struct so the
PDNA compiler does not depend on Panda3D.

The naming scheme for some files has been corrected, and some
classes have been moved to separate files.

compile.py (formerly parser.py) now uses argparse, rather than getopt.

compile.py adds the options:
    --verbose, -v
    --logfile, -l

Modified the PDNA header.

Overall cleanup.