indicate if an index is utilized in explain() step

It would be nice if the `explain()` step somehow was able to indicate whether a composite, mixed, or vertex-centric index will be utilized on the traversal.

Start by locating the explain() step and the code paths for composite, mixed, and vertex-centric index selection. Determine how explain() represents traversal planning, then define and implement output that clearly indicates whether each applicable index is utilized; add tests for the three index types.

Written by the indexing model from the issue text.