reject flags claimed by more than one option

[metsw24-max]

Duplicate flag matchers are accepted silently

Two options that register the same short or long flag currently parse without complaint (issue #117): Match() returns the first option in tree order, so the second can never be reached and any value meant for it is quietly dropped. ArgumentParser::Parse and Subparser::Parse now walk the reachable flags with GetAllFlags() and raise a UsageError (or set Error::Usage under ARGS_NOEXCEPT) naming the offending flag when one is claimed by more than one option. Going through GetAllFlags() keeps the check aware of command boundaries, so sibling subcommands may still reuse a name and a single option shared between groups does not report itself.

Regression tests cover both parsing modes and the subcommand case; they fail on master and pass with the change.

[Taywee]

This is a duplicate of #187, I'll check out both, but only one will be able to be merged.

[Taywee]

The implementation here is simple and correct, but I'd like evidence that it works across groups under the same parser (but not subcommands, as those separate their arguments).

[Taywee]

After poking at it a bit, I'm closing this in favor of the earlier one, as I've already interfaced with that one and it's been changed to handle what I wanted it to handle (which is to check on every added flag, rather than on parse).

Thanks for the PR; sorry this one didn't make it in.