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.go

@@ -1918,7 +1918,7 @@ func (s *RPCServer) GetExchangePairs(_ context.Context, r *gctrpc.GetExchangePai
 			return nil, err
 		}
 		if !assetTypes.Contains(a) {
-			return nil, fmt.Errorf("%w %v", asset.ErrNotSupported, a)
+			return nil, fmt.Errorf("%w %q", asset.ErrNotSupported, a)
 		}
 	}