Realm: Add standalone argument parser

[apryakhin]

Realm requires a more flexible version of the argument parser that provides users with comprehensive error feedback. After a series of discussions, we tentatively decided that the argument parser should become a standalone tool, possibly as a header-only library

[lightsighter]

As part of this it would be good if the parser could say which argument is badly formed when such cases occur. See this open merge request for context:

https://gitlab.com/StanfordLegion/legion/-/merge_requests/1312