full API for cop hint hashes

full API for cop hint hashes

Expose cop hint hashes as a type COPHH, with a cophh_* API which is a
macro layer over the refcounted_he_* API.  The documentation for cophh_*
describes purely API-visible behaviour, whereas the refcounted_he_*
documentation describes the functions mainly in terms of the
implementation.  Revise the cop_hints_* API, using the flags parameter
consistently and reimplementing in terms of cophh_*.  Use the cophh_*
and cop_hints_* functions consistently where appropriate.

[Modified by the committer to update two calls to
Perl_refcounted_he_fetch recently added to newPMOP.]