exchange/order/limits: Migrate to new package and integrate with exchanges (#1860)

* move limits, transition to key gen

* rollout NewExchangePairAssetKey everywhere

* test improvements

* self-review fixes

* ok, lets go

* fix merge issue

* slower value func,assertify,drop IsValidPairString

* remove binance reference for backtesting test

* Redundant nil checks removed due to redundancy

* Update order_test.go

* Move limits back into /exchanges/

* puts limits in a different box again

* SHAZBERT SPECIAL SUGGESTIONS

* Update gateio_wrapper.go

* fixes all build issues

* Many niteroos!

* something has gone awry

* bugfix

* gk's everywhere nits

* lint

* extra lint

* re-remove IsValidPairString

* lint fix

* standardise test

* revert some bads

* dupe rm

* another revert 360 mcgee

* un-in-revertify

* Update exchange/order/limits/levels_test.go

Co-authored-by: Adrian Gallagher <adrian.gallagher@thrasher.io>

* fix

* Update exchanges/binance/binance_test.go

HERE'S HOPING GITHUB FORMATS THIS CORRECTLY!

Co-authored-by: Gareth Kirwan <gbjkirwan@gmail.com>

* update text

* rn func, same line err gk4202000

---------

Co-authored-by: Adrian Gallagher <adrian.gallagher@thrasher.io>
Co-authored-by: Gareth Kirwan <gbjkirwan@gmail.com>

engine/rpcserver_test.go

@@ -117,12 +117,7 @@ func (f fExchange) GetOpenInterest(_ context.Context, k ...key.PairAsset) ([]fut
 	if len(k) > 0 {
 		return []futures.OpenInterest{
 			{
-				Key: key.ExchangePairAsset{
-					Exchange: f.GetName(),
-					Base:     k[0].Base,
-					Quote:    k[0].Quote,
-					Asset:    k[0].Asset,
-				},
+				Key:          key.NewExchangeAssetPair(f.GetName(), k[0].Asset, k[0].Pair()),
 				OpenInterest: 1337,
 			},
 		}, nil