Pend internal specs...

Two low-level tests of check_word_stem() are failing due to use of @alpha_word_list...

Pend them as they were used for initial testing only.
Add one_word_square for ease of live testing
Performance refactoring...

Generate/use a sub-list containing only words starting with the letter of a word-stem
Used in check_word_stem() to see if there's a possible word available.
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Minor refactoring...

Add line-feed to usage print command
Add internal timing display that occurs whenever there's a backtrack display
Fix error when backtracking...

Error introduced when updating the backtracking data.
Performance improvements...

Now memoize all word-stems examined, to limit unnecessary searches
  6-square process now visibly faster
Added various timing displays to track progress, especially for 6-squares
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