rbtree: removed offsetof define - it's already in blob.h
misc: move project specific includes to the top

This allows us to easily spot forgotten includes in header files.
Merge branch 'upstream' into devel
ci: use CC and CXX env variables

This adds clang compilation support
ci: set language to cpp so CC and CXX both get defined
l2hash: added const qualifier
l2hash: removed useless wrappers
l2hash: rename type to flavor to avoid confusion with columns
blob: added eblob_dump_id_len() race note
l2hash: murmur is a pure function
l2hash: remove obsolete comment
blob: __unused is used internaly by libc6 (pun intended)
blob: added __attribute_pure__ qualifier
blob: removed useless statement
data-sort: add speedup note
tests: check for returned size
tests: moved cpp test to tests/
tests: cpp: cleanups
ci: add eblob_cpp_test to autotests
tests: python: move python binding test to tests/