contracts: decipher oracle test contract

Make it a proper Go contract, opcodes are fun until you want to change
something in them. Part of #2412.

internal/contracts/oracle_contract/oracle.go

@@ -0,0 +1,24 @@
+package oraclecontract
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/interop/native/oracle"
+	"github.com/nspcc-dev/neo-go/pkg/interop/native/std"
+	"github.com/nspcc-dev/neo-go/pkg/interop/storage"
+	"github.com/nspcc-dev/neo-go/pkg/interop/util"
+)
+
+// RequestURL accepts a complete set of parameters to make an oracle request and
+// performs it.
+func RequestURL(url string, filter []byte, callback string, userData interface{}, gasForResponse int) {
+	oracle.Request(url, filter, callback, userData, gasForResponse)
+}
+
+// Handle is a response handler that writes response data to the storage.
+func Handle(url string, data interface{}, code int, res []byte) {
+	// ABORT if len(data) == 2, some tests use this feature.
+	if data != nil && len(data.(string)) == 2 {
+		util.Abort()
+	}
+	params := []interface{}{url, data, code, res}
+	storage.Put(storage.GetContext(), "lastOracleResponse", std.Serialize(params))
+}