Previously, the expressions given to the `-m` and `-k` options were
evaluated with `eval`. This causes a few issues:
- Python keywords cannot be used.
- Constants like numbers, None, True, False are not handled correctly.
- Various syntax like numeric operators and `X if Y else Z` is supported
unintentionally.
- `eval()` is somewhat dangerous for arbitrary input.
- Can fail in many ways so requires `except Exception`.
The format we want to support is quite simple, so change to a custom
parser. This fixes the issues above, and gives us full control of the
format, so can be documented comprehensively and even be extended in the
future if we wish.
Ran Benita