Please test: making leoAst.py stand-alone

[Edward K. Ream]

Recent revs add support for command-line arguments when running leo/core/leoAst.py directly:

...leo-editor\leo\core>python leoAst.py -h
usage:

    leoAst.py --help
    leoAst.py [--fstringify | --fstringify-diff | --orange | --orange-diff] PATHS
    leoAst.py --pytest ["-k ARGS"]
    leoAst.py --unittest [ARGS]

positional arguments:
  PATHS               directory or list of files

optional arguments:
  -h, --help          show this help message and exit
  --fstringify        leonine fstringify
  --fstringify-diff   show fstringify diff
  --orange            leonine Black
  --orange-diff       show orange diff
  --pytest "-k ARGS"  run pytest
  --unittest ARGS     run unittest

As a result:

1. Anyone can experiment with the Token Order classes without having to install Leo itself.

2. There is little need to "repackage" leoAst.py as a separate package. People can download just this file using git. One less thing for Matt to do :-)

Please test and report any problems.  All comments and suggestions welcome.

All tests (--unittest or --pytest) pass with Python 3.7. Some fail with python 3.8, due to new kinds of ast nodes.

Edward