add runtime_conditional and flow unary operator*

[lukevalenty]

TODO:

• ✅Better way to ensure predicates for the same condition are unique
• ✅operator&& for predicates
• ✅Better error message for edge/node predicate requirements
• ✅Documentation
• ✅More unit tests for edge/node predicate requirements
• ✅unary operator* for seq/par objects
• unary operator- for explicit references (can't be made into non-reference by unary +)
• ✅Message handler support
• ✅Indexed message handler tests
• runtime_condition support for cib::callback (do in follow-on PR)

[lukevalenty]

Passes CI in our downstream repo. 😎