- the helper functions for accessing twigs beneath a branch
(branch_twig_pos(), branch_twig_ptr(), etc) were somewhat confusing
to read, since several of them were implemented by calling other
helper functions. they now all show what they're really doing.
- branch_twigs_vector() has been renamed to simply branch_twigs().
- revised some unrelated comments in qp_p.h for clarity.
7f0242b8c7