facet by polygon [LUCENE-8368]

This can give some nice flexibility if you are working with search results on a map. Of course if everything about your use-case is static, its better to compute this up-front and index string values, but its not always the case. Also it can be helpful if your polygons are changing often, since you don't have to reindex.

Polygon2D already supports multipolygons, but today it only returns a boolean value. This patch adds a `find` method that returns the polygon that actually matched, or -1 if it doesn't match. `contains` is then just written as `find >= 0`.

Then we can solve the problem with just some sugar over the existing range faceting, as each multipolygon is just a range of ids coming back from `find` that correspond with it. e.g. if you were faceting by country, you might have \~200 countries with 100,000 total polygons, and polygons 22,000-32,000 correspond to Canada or whatever.

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Migrated from [LUCENE-8368](https://issues.apache.org/jira/browse/LUCENE-8368) by Robert Muir (@rmuir), updated Jun 25 2018
Attachments: [LUCENE-8368_hacky_grid.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-8368/LUCENE-8368_hacky_grid.patch) (versions: 3), [LUCENE-8368.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-8368/LUCENE-8368.patch)

Start with Polygon2D and its existing contains method, then inspect the range-faceting code that would consume polygon ids. Review the attached LUCENE-8368 patches for the proposed entry points and run the related spatial and faceting tests. Done means find identifies the matching polygon, contains preserves boolean behavior, and polygon-based faceting works for multipolygons.

Written by the indexing model from the issue text.