Add db package

Add own db (storage) package with db.Storage interface, with Memory & Leveldb
implementations, and with tests.

Adapted with several changes from
https://github.com/iden3/go-iden3-core/tree/v0.0.8/db

db/leveldb/leveldb_test.go

@@ -0,0 +1,57 @@
+package leveldb
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+
+	"github.com/iden3/go-merkletree/db"
+	"github.com/iden3/go-merkletree/db/test"
+	"github.com/stretchr/testify/require"
+)
+
+var rmDirs []string
+
+func levelDbStorage(t *testing.T) db.Storage {
+	dir, err := ioutil.TempDir("", "db")
+	rmDirs = append(rmDirs, dir)
+	if err != nil {
+		t.Fatal(err)
+		return nil
+	}
+	sto, err := NewLevelDbStorage(dir, false)
+	if err != nil {
+		t.Fatal(err)
+		return nil
+	}
+	return sto
+}
+
+func TestLevelDb(t *testing.T) {
+	test.TestReturnKnownErrIfNotExists(t, levelDbStorage(t))
+	test.TestStorageInsertGet(t, levelDbStorage(t))
+	test.TestStorageWithPrefix(t, levelDbStorage(t))
+	test.TestConcatTx(t, levelDbStorage(t))
+	test.TestList(t, levelDbStorage(t))
+	test.TestIterate(t, levelDbStorage(t))
+}
+
+func TestLevelDbInterface(t *testing.T) {
+	var db db.Storage //nolint:gosimple
+
+	dir, err := ioutil.TempDir("", "db")
+	require.Nil(t, err)
+	rmDirs = append(rmDirs, dir)
+	sto, err := NewLevelDbStorage(dir, false)
+	require.Nil(t, err)
+	db = sto
+	require.NotNil(t, db)
+}
+
+func TestMain(m *testing.M) {
+	result := m.Run()
+	for _, dir := range rmDirs {
+		os.RemoveAll(dir)
+	}
+	os.Exit(result)
+}