Keep a bitmask of what types of constraints we've seen in a set of impls

This will make it easier to pick a dispatcher appropriate for those constraints (https://github.com/cxw42/Sub-Multi-Tiny/issues/2#issuecomment-527517987). E.g., a bit each for:

- Do any impls use positional parameters?
- Are there at least two different positional arities in the set of impls?
- Do any impls use named parameters?
- Do any impls use type constraints?
- Do any impls use `where` clauses?

Start by reading the referenced comment on issue #2 and then locate the implementation that collects the set of multimethod implementations. Define completion as recording the listed positional, named, type-constraint, and where-clause properties, including whether positional arities differ, in a bitmask usable by dispatcher selection.

Written by the indexing model from the issue text.