[ruby/fiddle] Add TYPE_CONST_STRING and SIZEOF_CONST_STRING for "const char *"

Add rb_fiddle_ prefix to conversion functions.h to keep backward
compatibility but value_to_generic() isn't safe for TYPE_CONST_STRING
and not String src. Use rb_fiddle_value_to_generic() instead.

0ffcaa39e5

ext/fiddle/fiddle.h

@@ -116,6 +116,7 @@
 #define TYPE_FLOAT 7
 #define TYPE_DOUBLE 8
 #define TYPE_VARIADIC 9
+#define TYPE_CONST_STRING 10
 
 #define ALIGN_OF(type) offsetof(struct {char align_c; type align_x;}, align_x)