lib: rbtree: Add RB_FOR_EACH macro for iterative enumeration

Works mostly like the list enumeration macros.  Implemented by fairly
clever alloca trickery and some subtle "next node" logic.  More
convenient for many uses, can be early-exited, but has somewhat larger
code size than rb_walk().

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>