Move OP prototypes from pp_proto.h to proto.h

Move OP prototypes from pp_proto.h to proto.h

Make embed.pl fully responsible for generating prototypes and embedding macros
for pp_* and ck_* functions, placing them in embed.h and proto.h
opcode.pl no longer generates pp_proto.h
Remove the (effectively) duplicate explicit entries for (all but 2) ck_*
functions from embed.fnc

We can't actually remove pp_proto.h from the distribution *yet*, as
ExtUtils::MM_Unix and ExtUtils::MM_VMS have hardcoded lists of the installed
headers. Once this is resolved, we can.