Changing delta to 3 in Data.Map.
Changing delta to 3 in Data.Map.

Only possible values are 3 and 4. The value 3 has much faster inserts,
value 4 slightly faster deletes, so choosing 3.

Also changed the inequalities to rebalance only when one subtree
is _strictly_ larger than delta * the other one, to mimic the behaviour
from the proof (both from the Adams' and from the one to come).

shockkolate/containers,iu-parfunc/containers,hvr/containers,ekmett/containers,shockkolate/containers,meiersi/psqueues-old,Zigazou/containers,Zigazou/containers,shockkolate/containers,Zigazou/containers,iu-parfunc/containers,iu-parfunc/containers,ekmett/containers,hvr/containers,ekmett/containers,hvr/containers