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)
My initial approach separated the entire metadex into its own set of files, but the end result was not so.