Write sorting::introsortLoop without recursion

[Ben Corbett]

Currently sorting::introsortLoop uses recursion, nvcc allocates space for the stack at compile time and it issues a warning that it can't statically determine the stack size. This isn't an pressing issue but it does bring up the possibility that we blow through the stack (unlikely though since it has a controlled recursion level). However if it can be rewritten without recursion and without sacrificing performance this would be nice.