Add public API to iterate over L2/L3 data

[tommyip]

Fixes #208

[tommyip]

Usage:

use serum_dex::utils::{OrderBook, Level};

let bids_slab = market_state.load_bids_mut(&bids_account_info)?;
let orderbook = OrderBook::new(Side::Bid, &bids_slab);

let eg1 = orderbook.levels();
let eg2: Vec<Level> = orderbook.levels().take(10).collect();

[dovahcrow]

Can we have Default derived for Level?

[tommyip]

Added. The maintainers doesn't seem to be interested in this PR though.

[dovahcrow]

Added. The maintainers doesn't seem to be interested in this PR though.

Thank you! Yeah I hope this PR can be merged soon. I've been using your branch for a long time. It is very useful! I appreciate your work here.