Terms.toStringDebug [LUCENE-7361]

While fixing #8394, MemoryIndex.toString(), I thought MemoryIndex shouldn't need it's own debug toString() impl for its Terms when there could be a generic one. So here I propose that we create a Terms.toStringDebug(Appendable result, int charLimit, String indent) or some-such but probably not override toString() for obvious reasons. Maybe also have this on Fields() that simply loops and calls out to the one on Terms.

The format is debatable.

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Migrated from [LUCENE-7361](https://issues.apache.org/jira/browse/LUCENE-7361) by David Smiley (@dsmiley), updated Jun 30 2016
Attachments: [MemoryIndexToString.java](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-7361/MemoryIndexToString.java)

Start with the attached MemoryIndexToString.java and the existing MemoryIndex.toString() implementation, then inspect the Terms and Fields entry points. Compare the proposed generic Terms debug method with the possible Fields delegation and clarify the output format. Done means the scope and format are agreed and the resulting debug representation is covered by relevant tests.

Written by the indexing model from the issue text.