Optimizations + some improvements (#105)

* Optimizations + some improvements

- optimized pkg/core/storage.HeaderHashes
- optimized pkg/rpc.performRequest (used json.Encoder)
- fixes for pkg/util.ReadVarUint and pkg/util.WriteVarUint
- optimized and fix fixed8 (Fixed8DecodeString / MarshalJSON) + tests
- optimized and fix uint160 (Bytes / Uint160DecodeString / Equal / MarshalJSON) + tests
- optimized and fix uint256 (Bytes / Equal / MarshalJSON) + tests
- preallocate for pkg/vm.buildStackOutput
- add go.mod / go.sum

* update version

pkg/rpc/client.go

@@ -83,18 +83,21 @@ func NewClient(ctx context.Context, endpoint string, opts ClientOptions) (*Clien
 }
 
 func (c *Client) performRequest(method string, p params, v interface{}) error {
-	r := request{
-		JSONRPC: c.version,
-		Method:  method,
-		Params:  p.values,
-		ID:      1,
-	}
+	var (
+		r = request{
+			JSONRPC: c.version,
+			Method:  method,
+			Params:  p.values,
+			ID:      1,
+		}
+		buf = new(bytes.Buffer)
+	)
 
-	b, err := json.Marshal(r)
-	if err != nil {
+	if err := json.NewEncoder(buf).Encode(r); err != nil {
 		return err
 	}
-	req, err := http.NewRequest("POST", c.endpoint.String(), bytes.NewReader(b))
+
+	req, err := http.NewRequest("POST", c.endpoint.String(), buf)
 	if err != nil {
 		return err
 	}