Move prototype definition of SvPV_helper to embed.fnc

It's usually a bad idea to try to work around a limitation in common
code by copy-pasting and then modifiying to taste.  Fixes/improvements
to the common code rarely get propagated to the outlier.

I wrote code in 1ef9039 that did just this for the prototype
definition of SvPV_helper, because the place where it really belongs,
embed.fnc, couldn't (and still doesn't) handle function pointers as
arguments (patches welcome).

I should have at least added a comment to the common code noting the
existence of this outlier.

It turns out that that limitation can be worked around by declaring a
typedef of the pointer, and then using that in embed.fnc.

That's what this commit does.

This commit removes the final instance of duplicating the work of
embed.fnc in the core, except for some in the regex system whose
comments say the reason is to avoid making a typedef public.  I haven't
investigated these further.

Author: khwilliamson

embed.fnc

@@ -3374,6 +3374,13 @@ CRdmp	|char * |sv_2pvbyte_nolen					\
 Adp	|char * |sv_2pv_flags	|NN SV * const sv			\
 				|NULLOK STRLEN * const lp		\
 				|const U32 flags
+Cip	|char * |SvPV_helper	|NN SV * const sv				\
+				|NN STRLEN * const lp				\
+				|const U32 flags				\
+				|const PL_SvPVtype type 			\
+				|NN Perl_SvPV_helper_non_trivial_t  non_trivial \
+				|const bool or_null				\
+				|const U32 return_flags
 Cdmp	|char * |sv_pvn_force	|NN SV *sv				\
 				|NULLOK STRLEN *lp
 Adp	|char * |sv_pvn_force_flags					\

embed.h

@@ -109,6 +109,7 @@
 # define SvNV(a)                                Perl_SvNV(aTHX_ a)
 # define SvNV_nomg(a)                           Perl_SvNV_nomg(aTHX_ a)
 # define SvPVXtrue(a)                           Perl_SvPVXtrue(aTHX_ a)
+# define SvPV_helper(a,b,c,d,e,f,g)             Perl_SvPV_helper(aTHX_ a,b,c,d,e,f,g)
 # define SvREFCNT_dec_ret_NULL(a)               Perl_SvREFCNT_dec_ret_NULL(aTHX_ a)
 # define SvTRUE(a)                              Perl_SvTRUE(aTHX_ a)
 # define SvTRUE_NN(a)                           Perl_SvTRUE_NN(aTHX_ a)

proto.h

@@ -1958,16 +1958,8 @@ typedef enum {
     SvPVbyte_pure_type_
 } PL_SvPVtype;
 
-START_EXTERN_C
-
-/* When this code was written, embed.fnc could not handle function pointer
- * parameters; perhaps it still can't */
-#ifndef PERL_NO_INLINE_FUNCTIONS
-PERL_STATIC_INLINE char*
-Perl_SvPV_helper(pTHX_ SV *const sv, STRLEN *const lp, const U32 flags, const PL_SvPVtype type, char * (*non_trivial)(pTHX_ SV *, STRLEN * const, const U32), const bool or_null, const U32 return_flags);
-#endif
-
-END_EXTERN_C
+typedef char * (*Perl_SvPV_helper_non_trivial_t)(pTHX_ SV *, STRLEN * const,
+                                                       const U32);
 
 /* This test is "is there a cached PV that we can use directly?"
  * We can if