Populate additional fields in Price struct to be compatible with Pyth SDK

[partiallysorted]

Added the following to initialize:

    price_oracle.magic = MAGIC;
    price_oracle.ver = VERSION;
    price_oracle.atype = AccountType::Price;

Added the following to set_price:

    price_oracle.curr_slot = clock.slot;
    price_oracle.valid_slot = clock.slot;