Add RandomX and RandomARQ as submodule
Hi @oliverw 
What do you think about adding randomX and randomARQ as git submodule to the repository? Currently the build script clones these repositories on each run and it slows down builds while developing and testing quite a bit. 

Having these repos as submodules would also ensure that miningcore could still be compiled (at least locally) even when one of these repos get deleted. 
