Use FooBAR convention for new pad macros

Use FooBAR convention for new pad macros

After a while, I realised that it can be confusing for PAD_ARRAY and
PAD_MAX to take a pad argument, but for PAD_SV to take a number and
PAD_SET_CUR a padlist.

I was copying the HEK_KEY convention, which was probably a bad idea.
This is what we use elsewhere:

   TypeMACRO
   ----=====
     AvMAX
    CopFILE
   PmopSTASH
  StashHANDLER
 OpslabREFCNT_dec

Furthermore, heks are not part of the API, so what convention they use
is not so important.

So these:

    PADNAMELIST_*
    PADLIST_*
    PADNAME_*
    PAD_*

are now:

    Padnamelist*
    Padlist*
    Padname*
    Pad*