Added sketch for VM

DO NOT MERGE
This is just a sketch.
Please comment.
My rationale was short string packing.
E.g. if string length is less then sizeof(v7_strlen_t), then `char *buf` is actually `char buf[sizeof(v7_strlen_t)]`
But apparently it'd work with any field order.
Reverting to the usual order - and the whole topic of string packing yet to be discussed.