Make `OrderBookTier` derive `Serialize`

[gitmalong]

For testing purposes it would be handy if OrderBookTier derives Serialize so one can quickly get some test data from a real OrderBookTier.

[gitmalong]

Make PairInfo derive Clone would also be useful.

e.g. now I can only derive a reference from HashMap<String, PairInfo>

fn find_asset_pair(&self, symbol_a: &str, symbol_b: &str) -> Option<&PairInfo> {
    self.asset_pairs.values().find(|pair| {
        pair.base == symbol_a && pair.quote == symbol_b
            || pair.base == symbol_b && pair.quote == symbol_a
    })
}

[gmosx]

Hm, I will add those temporarily, but probably I should make serialization of api types an optional feature of the crate, in a future PR.