add support for other *pvn* to *pvs* macros/APIs

[tonycoz]

There's plenty to do here.

My only issue is avoiding duplicating most of the code.

I suspect I can do it via template arguments, but char[] template parameters are new/not available (C++20? 23?).

I may need to use inheritance to make it work instead, but that's pretty ugly.

Preferably I'd supply the parameters as constructor arguments, but this is all created by a factory we don't control as far as I can see.

[tonycoz]

Others to add:

• [ ] cophh_fetch_pvs
• [ ] cophh_exists_pvs
• [ ] cophh_store_pvs
• [ ] cophh_delete_pvs
• [x] sv_catpvs_flags
• [x] sv_catpvs_nomg
• [x] sv_catpvs
• [ ] sv_catpvs_mg
• [x] sv_setpvs_mg
• [ ] sv_setref_pvs
• [x] savepvs
• [ ] savesharedpvs
• [ ] gv_stashpvs
• [ ] gv_fetchpvs
• [ ] lex_stuff_pvs
• [x] get_cvs
• [ ] hv_existss
• [ ] refcounted_he_fetch_pvs
• [ ] refcounted_he_new_pvs
• [ ] pad_add_name_pvs
• [ ] pad_findmy_pvs

[tonycoz]

We can control the factory and now do, and using that to supply the parameters.