Optimized FinderIndexerHelper->stem() method

[frankmayer]

Summary of Changes

• Optimized FinderIndexerHelper->stem() method now being five times faster on its own running time (under PHP 7.0, probably even faster under 5.6).
• One type safe comparison

Testing Instructions

• Code review
• Though no change in behavior is expected, indexing and search-tests with English and non-English content should probably be done.

Documentation Changes Required

None

[zero-24]

I have tested this item ✅ successfully on 3cb3583

Tested at #jc17de 😄

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[widmann-it]

I have tested this item ✅ successfully on 3cb3583

Successfully tested on #jc17de

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/13480.}

[gunthelm2000]

I have tested this item ✅ successfully on 3cb3583

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/13480.}

[gunthelm2000]

Successfully tested on #jc17de

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/13480.}