P.within on an indexed property does not use the index

For example, the Gremlin traversal:
```
g.V().has("INDEXED_PROPERTY", P.within("foo", "bar"));
```
takes time linear in the length of the graph, whereas:
```
g.V().has("INDEXED_PROPERTY", "foo");
```
is very fast.

It seems like the former query could use the index to improve performance.

Start by tracing how the Gremlin traversal with P.within is handled for indexed properties, then compare it with the direct equality lookup described in the issue. Confirm the relevant index path is used for multiple values and measure the traversal against the examples; done means the P.within query no longer scans the graph linearly.

Written by the indexing model from the issue text.