Integrate UsethlessMiner for CPU-based KAPOW and ETHHASH mining
**Describe the bug**
Feature request: Integrate https://github.com/Chainfire/UselethMiner to utilize CPU to mine ETHHASH and KAPOW. 

**To Reproduce**
n/a

**Expected behavior**
Be able to compile xmrig with usethlessminer backend for cpu mining of KAPOW and ETHHASH

**Required data**
source: **https://github.com/Chainfire/UselethMiner**
**Additional context**
Geth would be another route, but useleth seems to be fastest. Making note of this to work on pull request and fiddling around if there is time. 
