Add optimizer constraint that number of missing letters must be equal to some hidden word length

[jonadsimon]

Currently optimization can fail when the number of missing letters does not equal the length of any hidden word.

Add a constraint to the optimizer to enforce that the number of missing letters must equal some missing word length

Print a message when this condition is triggered so we know whether to increment vs decrement the packing constant

[jonadsimon]

Duplicate: https://github.com/jonadsimon/wonder-words-generator/issues/23