Fix the bug of testing prefixes and planting

Previously the functions `is_prefix` and `plant` did not take the
situation of packed nodes into considerations.  That was because I
only dealt with non-packed nodes in the past: the fragment to test for
prefixes and for planting did not intersect the packed nodes in the
forest, and the grammar is so simple that the fragments do not contain
packed nodes.

Then a test revealed this situation, so I have to fix this lack of
considerations now.  This commit attempts to fix this issue.

From the newly added unit-tests, it seems that this fix works.  :)

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