rpc: move all top*FromStack client functions to a separate file

Some of them are shared between multiple RPC client APIs, so let's keep
them in a separate place for better maintainability.

pkg/rpc/client/helper.go

@@ -0,0 +1,77 @@
+package client
+
+import (
+	"crypto/elliptic"
+	"errors"
+	"fmt"
+
+	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
+	"github.com/nspcc-dev/neo-go/pkg/rpc/response/result"
+	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
+)
+
+// getInvocationError returns an error in case of bad VM state or empty stack.
+func getInvocationError(result *result.Invoke) error {
+	if result.State != "HALT" {
+		return fmt.Errorf("invocation failed: %s", result.FaultException)
+	}
+	if len(result.Stack) == 0 {
+		return errors.New("result stack is empty")
+	}
+	return nil
+}
+
+// topBoolFromStack returns the top boolean value from stack.
+func topBoolFromStack(st []stackitem.Item) (bool, error) {
+	index := len(st) - 1 // top stack element is last in the array
+	result, ok := st[index].Value().(bool)
+	if !ok {
+		return false, fmt.Errorf("invalid stack item type: %s", st[index].Type())
+	}
+	return result, nil
+}
+
+// topIntFromStack returns the top integer value from stack.
+func topIntFromStack(st []stackitem.Item) (int64, error) {
+	index := len(st) - 1 // top stack element is last in the array
+	bi, err := st[index].TryInteger()
+	if err != nil {
+		return 0, err
+	}
+	return bi.Int64(), nil
+}
+
+// topPublicKeysFromStack returns the top array of public keys from stack.
+func topPublicKeysFromStack(st []stackitem.Item) (keys.PublicKeys, error) {
+	index := len(st) - 1 // top stack element is last in the array
+	var (
+		pks keys.PublicKeys
+		err error
+	)
+	items, ok := st[index].Value().([]stackitem.Item)
+	if !ok {
+		return nil, fmt.Errorf("invalid stack item type: %s", st[index].Type())
+	}
+	pks = make(keys.PublicKeys, len(items))
+	for i, item := range items {
+		val, ok := item.Value().([]byte)
+		if !ok {
+			return nil, fmt.Errorf("invalid array element #%d: %s", i, item.Type())
+		}
+		pks[i], err = keys.NewPublicKeyFromBytes(val, elliptic.P256())
+		if err != nil {
+			return nil, err
+		}
+	}
+	return pks, nil
+}
+
+// top string from stack returns the top string from stack.
+func topStringFromStack(st []stackitem.Item) (string, error) {
+	index := len(st) - 1 // top stack element is last in the array
+	bs, err := st[index].TryBytes()
+	if err != nil {
+		return "", err
+	}
+	return string(bs), nil
+}