Perl_sv_upgrade_fresh added to inline.h

When a new SV is created and upgraded to a type known at compile time,
using the general-purpose upgrade function (sv_upgrade) is clunky.
Specifically, it is too big to be inlined, contains many branches that
can logically be resolved at compile time for known start & end types,
and the lookup of the correct body_details struct may add CPU cycles.

This commit does two things:
* adds Perl_sv_upgrade_fresh for use with SVs of type SVt_NULL
* moves structure, macro and function definitions required to allow
  this new function to reside in inline.h

Subsequent commits will make use of this new function.

Author: richardleach

embed.fnc

@@ -1947,6 +1947,7 @@ ApdMb	|void	|sv_unref	|NN SV* sv
 Apd	|void	|sv_unref_flags	|NN SV *const ref|const U32 flags
 Cpd	|void	|sv_untaint	|NN SV *const sv
 Apd	|void	|sv_upgrade	|NN SV *const sv|svtype new_type
+Cpdix	|void	|sv_upgrade_fresh	|NN SV *const sv|svtype new_type
 ApdMb	|void	|sv_usepvn	|NN SV* sv|NULLOK char* ptr|STRLEN len
 Apd	|void	|sv_usepvn_flags|NN SV *const sv|NULLOK char* ptr|const STRLEN len\
 				|const U32 flags

embed.h

@@ -717,6 +717,7 @@
 #define sv_unref_flags(a,b)	Perl_sv_unref_flags(aTHX_ a,b)
 #define sv_untaint(a)		Perl_sv_untaint(aTHX_ a)
 #define sv_upgrade(a,b)		Perl_sv_upgrade(aTHX_ a,b)
+#define sv_upgrade_fresh(a,b)	Perl_sv_upgrade_fresh(aTHX_ a,b)
 #define sv_usepvn_flags(a,b,c,d)	Perl_sv_usepvn_flags(aTHX_ a,b,c,d)
 #define sv_utf8_decode(a)	Perl_sv_utf8_decode(aTHX_ a)
 #define sv_utf8_downgrade_flags(a,b,c)	Perl_sv_utf8_downgrade_flags(aTHX_ a,b,c)