add complex.hlsl

[nahiim]

Description

Start drafting complex.hlsl

[devshgraphicsprogrammingjenkins]

[CI]: Can one of the admins verify this patch?

[devshgraphicsprogramming]

I see a certain ladder of features, and each should have their own branch (so I can merge in order without having all comments addressed):

1. having a complex_t struct to use
2. subgroup::fft utilities for DIT and DIF
3. workgroup::fft utilities for DIT and DIF
4. a way to do multiple input items per-workgroup, i.e. do a 2048 sized FFT with a 512 workgroup while having the option of only using enough shared memory for 512 FFT + auxilary accessor
5. optimizations for FFT real signals
6. resurrect the nbl::ext and 1 or both of the old FFT examples to test with

They can branch off each other.

P.S. Let me know if you need "runtime" transcendental/special functions for complex_t then we can look into doing up our tgmath.hlsl