Move string type to runtime in separate file

src/runtime/string.go

@@ -0,0 +1,20 @@
+package runtime
+
+// This file implements functions related to Go strings.
+
+type _string struct {
+	length lenType
+	ptr    *uint8
+}
+
+func stringEqual(x, y string) bool {
+	if len(x) != len(y) {
+		return false
+	}
+	for i := 0; i < len(x); i++ {
+		if x[i] != y[i] {
+			return false
+		}
+	}
+	return true
+}