IMPORT: import cebtree (compact elastic binary trees)

IMPORT: import cebtree (compact elastic binary trees)

This is an import of the compact elastic binary trees at commit
a9cd84a ("OPTIM: descent: better prefetch less and for writes when
deleting")

These will be used to replace certain lists (and possibly certain
tree nodes as well). They're as fast (or even faster) than ebtrees
for lookups, as fast for insertion and slower for deletion, and a
node only uses 2 pointers (like a list).

The only changes were cebtree.h where common/tools.h was replaced
with ebtree.h which we already have and already provides the needed
functions and macros, and the addition of a wrapper cebtree-prv.h in
src/ to redirect to import/cebtree-prv.h.