Add persistence md

Adds persistence

lots to discuss... I'm not 100% sure the way i made get_prices and get_indexes available to mastercore.cpp is the best way (using mastercore::)

i reused some of the dex variable names (but added comments on how metadex uses them)

Most loading/saving logic came from mastercore_dex.cpp, cleaned up a little

no way to set the action, so added another method to do it (setAction)
Ah I see. I just fear that the next time there is any change to either one of both structures (which look similar, but describe different things), it might require significant refactoring - and those circumstances will make it more difficult to push change at all. I mention this in particular, because many spec limitations or restrictions were justified with  "it's too complicated now, we can always do it later". This is of course no critique of this PR or anything, but just an observation that has probably a broader scope. :)