LT-22353: Change Hermit Crab to allow merge and split rules

[jtmaxwell3]

This fixes https://jira.sil.org/browse/LT-22353. We want to add merge rules (2 -> 1) and split rules (1 -> 2) in the phonology. It turns out that NarrowAnalysisRewriteRuleSpec implemented merge rules, but the code had never been tested and there was a bug when the right hand side had anything in it. I added unit tests and fixed the bug. It also turned out that NarrowAnalysisRewriteRuleSpec worked for expanding as well as narrowing, so I added unit tests for expanding and code to use NarrowAnalysisRewriteRuleSpec in this case. There is no need to test for an infinite loop (see EpenthesisSynthesisRewriteSubruleSpec) when expanding since a rule cannot apply to its own output.

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This change is 

[ddaspit]

@ddaspit reviewed 4 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @jtmaxwell3).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.67%. Comparing base (58392e4) to head (35e728c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
+ Coverage   72.61%   72.67%   +0.06%     
==========================================
  Files         423      423              
  Lines       35992    36009      +17     
  Branches     4965     4965              
==========================================
+ Hits        26134    26169      +35     
+ Misses       8760     8745      -15     
+ Partials     1098     1095       -3     

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