Allow inserts with more than one hint

Hints improve performance. Sometimes we have more than one hint available before insert, and not using them all is a waste of time and power.
I figure the insert would have to be on the multi_index object itself and not a specific index.

Start by reading the multi_index insertion API and the existing handling of insertion hints. Determine how multiple available hints should be supplied at the multi_index level, then verify that inserts can use all applicable hints without changing expected behavior.

Written by the indexing model from the issue text.