Change Add to PriorityUnion

[jtmaxwell3]

I fixed a performance bug by changing AnalysisAffixProcessRule.Apply to use PriorityUnion instead of Add. Add acts like Union, PriorityUnion is more restrictive. Also, SynthesisAffixProcessRule uses PriorityUnion instead of Add in the other direction, so this makes AnalysisAffixProcessRule more parallel to SynthesisAffixProcessRule.

If I have a sequence of derivation rules that change the category like [CAT:A] => [CAT:B], [CAT:B] => [CAT:A], followed by [CAT:B] => [CAT:C], then the syntactic feature structure of the input of the third rule will be [CAT:{A, B}] if I use Add, and the third rule will not be filtered when it checks whether the input can unify with [CAT:B]. If I use PriorityUnion, then the syntactic feature structure of the input of the third rule will be [CAT:A], and the third rule will be filtered when it checks whether the input can unify with [CAT:B].

This performance bug only shows up when there are at least three derivational affixes. But words that are slow to parse try many long sequences of derivational affixes. Filtering these long sequences can make a big difference.

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This change is 

[ddaspit]

It would be great if you could add a unit test to capture scenario you laid out.

@ddaspit reviewed 1 file and all commit messages, and made 2 comments.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on jtmaxwell3).

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src/SIL.Machine.Morphology.HermitCrab/MorphologicalRules/AnalysisAffixProcessRule.cs line 59 at r1 (raw file):

                {
                    if (!_rule.RequiredSyntacticFeatureStruct.IsEmpty)
                        outWord.SyntacticFeatureStruct.PriorityUnion(_rule.RequiredSyntacticFeatureStruct);

This change should also apply to compounding rules as well.