Fix qp-trie refcounting mistake

Fix qp-trie refcounting mistake

The error occurred when:

  * The bump chunk was re-used across multiple write transactions.
    In this situation the bump chunk is marked immutable, but the
    immutable flag is disregarded for cells after the fender, which
    were allocated in the current transaction.

  * The bump chunk fills up during an insert operation, so that the
    enlarged twigs vector is allocated from a new bump chunk.

  * Before this happened, we should have (but didn't) made the twigs
    vector mutable. This would have adjusted its refcounts as necessary.

  * However, moving to a new bump chunk has a side effect: twigs that
    were previously considered mutable because they are after the
    fender become immutable.

  * Because of this, the old twigs vector was not destroyed as expected.

  * So leaves were duplicated without their refcounts being increased.

The effect is that the refcounts were lower than they should have
been, and underflowed. The tests failed to check for refcount
underflow, so this mistake was detected much later than it ideally
could have been.

After the fix, it is now correct not to ensure the twigs are mutable,
because they are about to be copied to a larger vector. Instead, we
need to find out whether `squash_twigs()` destroyed the old twigs, and
adjust the refcounts accordingly.