add oracle_price_lots fn

[skrrb]

currently oracle_price is converted to oracle_price_lots inside book’s new_order

openbook-v2/programs/openbook-v2/src/state/orderbook/book.rs

Line 82 in 706186b

let oracle_price_lots = if let Some(oracle_price) = oracle_price {

hence, when placing multiple orders, the same (possible costly -i80f48 math-) operation will be repeated several times. This PR makes the calculation before the new_order loop so it's only computed once per ix with the intention of saving some CUs