Further additions of std::string_view

- corefile.cpp, fileio.cpp: Change puts to take a std::string_view parameter
- rendlay.cpp: Use std::string_view instead of bare pointers in various functions
- vecstream.h: Add std::string_view conversion operator to obtain output buffer without needing to make it a C string with explicit null termination
- xmlfile.cpp: Add get_attribute_string_ptr method that distinguishes between empty strings and absent attributes without falling back to C strings

src/emu/fileio.cpp

@@ -622,7 +622,7 @@ u32 emu_file::write(const void *buffer, u32 length)
 //  puts - write a line to a text file
 //-------------------------------------------------
 
-int emu_file::puts(const char *s)
+int emu_file::puts(std::string_view s)
 {
 	// write the data if we can
 	if (m_file)

src/emu/fileio.h

@@ -179,7 +179,7 @@ class emu_file
 
 	// writing
 	u32 write(const void *buffer, u32 length);
-	int puts(const char *s);
+	int puts(std::string_view s);
 	int vprintf(util::format_argument_pack<std::ostream> const &args);
 	template <typename Format, typename... Params> int printf(Format &&fmt, Params &&...args)
 	{

src/emu/image.cpp

@@ -169,7 +169,7 @@ int image_manager::write_config(emu_options &options, const char *filename, cons
 	if (filerr == osd_file::error::NONE)
 	{
 		std::string inistring = options.output_ini();
-		file.puts(inistring.c_str());
+		file.puts(inistring);
 		retval = 0;
 	}
 	return retval;