lib/trees: improve bintrees efficiency

First, comment potential slow asserts in the hot-path.
These asserts are needed only to check that the implementation is
correct.

Second, use a single `<=>` instead of doing multiple comparisons.
So reduce the total number of comparisons.

On a real instance (IA-related problems), the gain is the following:

RBTreeMap, 8738 insertions, 24855 accesses
Before: 2.54s
After: 0.07s (so, in the noise)

Signed-off-by: Jean Privat <jean@pryen.org>