Remove many superfluous 'open' indicated by ocamlc -w +33

 With ocaml 4.01, the 'unused open' warning also checks the mli :-)

 Beware: some open are reported as useless when compiling with camlp5,
 but are necessary for compatibility with camlp4. These open are now
 marked with a comment.
Remove some dead-code (thanks to ocaml warnings)

 The removed code isn't used locally and isn't exported in the signature
Lazily computed hash in KerName.t.
Canary testing absence of generic equality for KerNames
Custom canary