Improve performance of string conversion

Convert to string with one call to snprintf and no string concatenations
instead of 5 calls to snprintf and 8 concatenations. Microbenchmarks
indicate that this is approximately 17% faster (1.17 times as fast).

src/guid.cpp

@@ -113,27 +113,16 @@ bool Guid::isValid() const
 // convert to string using std::snprintf() and std::string
 std::string Guid::str() const
 {
-	char one[10], two[6], three[6], four[6], five[14];
-
-	snprintf(one, 10, "%02x%02x%02x%02x",
-		_bytes[0], _bytes[1], _bytes[2], _bytes[3]);
-	snprintf(two, 6, "%02x%02x",
-		_bytes[4], _bytes[5]);
-	snprintf(three, 6, "%02x%02x",
-		_bytes[6], _bytes[7]);
-	snprintf(four, 6, "%02x%02x",
-		_bytes[8], _bytes[9]);
-	snprintf(five, 14, "%02x%02x%02x%02x%02x%02x",
+	constexpr size_t guidSize = sizeof("01234567-0123-0123-0123-0123456789ab");
+	char out[guidSize];
+	snprintf(out, guidSize, "%02x%02x%02x%02x-%02x%02x-%02x%02x-%02x%02x-%02x%02x%02x%02x%02x%02x",
+		_bytes[0], _bytes[1], _bytes[2], _bytes[3],
+		_bytes[4], _bytes[5],
+		_bytes[6], _bytes[7],
+		_bytes[8], _bytes[9],
 		_bytes[10], _bytes[11], _bytes[12], _bytes[13], _bytes[14], _bytes[15]);
-	const std::string sep("-");
-	std::string out(one);
 
-	out += sep + two;
-	out += sep + three;
-	out += sep + four;
-	out += sep + five;
-
-	return out;
+	return std::string(out, guidSize - 1);
 }
 
 // conversion operator for std::string