Improved OpenCL kernel

Major changes since V0.2.1:

• CI artifacts for windows #22
• Benchmark action #23
• Stats action #25
• OpenCL kernel rewrite for a large performance improvement #18

Support Nano node V21, protocol V18

Base difficulty updated to 0xfffffff800000000. Not important if used as a work peer from the node, as the difficulty is always explicitly requested.

Others:

• Removed support for launch flag --beta
• Shuffle queue of work using launch flag --shuffle
• ocl updated to 0.19.3