it: use uint32_t instead of enum (for now)
test: set data flag on iteration
it: improved logging
test: rework iterator test, added flow control
it: allow flow control of iterators
it: improve logging
it: move flow control to separate subroutine
it: added sanity and comments
it: prettify comments
rbtree: clang defines offsetof in stddef.h
features: bump _XOPEN_SOURCE to 600

This enables support for SUS sixth revision.
Also addd it where it was missed. This cleanups build under lucid.

TODO: Move it to features.h header
TODO: Bump _POSIX_C_SOURCE 200809L
TODO: Bump _XOPEN_SOURCE to 700
TODO: Switch to -std=c99?
history: fixed return value handling
it: python: simplified code