Merge branch 'ab/designated-initializers' into ab/designated-initializers-more

* ab/designated-initializers: cbtree.h: define cb_init() in terms of CBTREE_INIT *.h: move some *_INIT to designated initializers *.h _INIT macros: don't specify fields equal to 0 *.[ch] *_INIT macros: use { 0 } for a "zero out" idiom submodule-config.h: remove unused SUBMODULE_INIT macro