qpkg: converted dumb binary trees to red-black trees (in progress)

This is a change of the underlying mechanism but it's not polished or
optimized yet. The compare functions haven't been updated, so they work
but produce compiler warnings because of type mismatches.

- Makefile (OBJS): added jrb.o
- id.h, id.c: use jrb instead of own dumb binary trees
- TODO: brag about the efficiency improvement

qpkg/TODO

@@ -8,6 +8,10 @@
 
   So the tree is clearly degenerated.
 
+  Update: after switching to red-black trees, we get only 1497604 calls
+  to comp_id. This is 130% of the "good case" estimate above. Insertion
+  of a new node is currently done with two lookup, so we'll get rid of
+  some more lookups after further optimization.
 
 - check whether introducing a new package would cause a conflict