core: implement MPT Billet structure for MPT restore

MPT restore process is much simpler then regular MPT maintaining: trie
has a fixed structure, we don't need to remove or rebuild MPT nodes. The
only thing we should do is to replace Hash nodes to their unhashed
counterparts and increment refcount. It's better not to touch the
regular MPT code and create a separate structure for this.

pkg/core/mpt/helpers_test.go

@@ -0,0 +1,20 @@
+package mpt
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestToNibblesFromNibbles(t *testing.T) {
+	check := func(t *testing.T, expected []byte) {
+		actual := fromNibbles(toNibbles(expected))
+		require.Equal(t, expected, actual)
+	}
+	t.Run("empty path", func(t *testing.T) {
+		check(t, []byte{})
+	})
+	t.Run("non-empty path", func(t *testing.T) {
+		check(t, []byte{0x01, 0xAC, 0x8d, 0x04, 0xFF})
+	})
+}