Implement dlib_vector_length to calculate the euclidean distance of the vectors

It not use the 'length' native versions of dlib, due I have to convert the
array to the vector, which will take the same time, and then I will use the
native method.

src/vector.h

@@ -0,0 +1,10 @@
+#ifndef PHP_DLIB_VECTOR_H
+#define PHP_DLIB_VECTOR_H
+
+ZEND_BEGIN_ARG_INFO_EX(dlib_vector_length_arginfo, 0, 0, 2)
+    ZEND_ARG_INFO(0, x_arg)
+    ZEND_ARG_INFO(0, y_arg)
+ZEND_END_ARG_INFO()
+PHP_FUNCTION(dlib_vector_length);
+
+#endif //PHP_DLIB_VECTOR_H