Make problem parsing tolerant of delimiter whitespace

[sylvesterkaczmarek]

Fixes #166.

Makes the problem-text parser tolerant of ordinary whitespace variations around ;, =, ,, and ?. It also avoids treating a trailing semicolon before the goal as an empty clause, while preserving the empty dependency-clause form used by defs.txt.

The regression test compares irregularly spaced input with the existing canonical form.

Checks:

• problem_test.py: 3 tests passed
• direct checks for trailing-semicolon goal parsing and the empty dependency clause
• python3 -m compileall problem.py problem_test.py
• git diff --check