try to refactor main stem() loop of Hunspell stemmer [LUCENE-5829]

As pointed out by Ryan on #6888

This is the most complicated part, basically intersecting against 3 FSTs. But currently its premature to try to fix: its incomplete. it doesn't yet implement all the features it needs like decomposition. After we do this, we should see if we can refactor logic (e.g. somehow share affix compatibility better and so on).

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Migrated from [LUCENE-5829](https://issues.apache.org/jira/browse/LUCENE-5829) by Robert Muir (@rmuir)

Start by reading the context in #6888 and the Hunspell stemmer's main stem() loop, which currently intersects three FSTs. Confirm that decomposition and the other required features are implemented before attempting the refactor; done means the completed logic has a clearer structure with shared affix compatibility where appropriate.

Written by the indexing model from the issue text.