1.4.0

CHANGES:

• fixed performance bugs in weak hash tables implementation,
  by back porting some old fixes from OCaml's Weak module
  (reported by Edwin Török)
• improved equality functions in Hset and Hmap
  (contributed by Dorian Lesbre)
• a lot of missing functions in Hset and Hmap wrt OCaml's Set and Map,
  with the notable exception of to_seq_rev
  (contributed by Dorian Lesbre)