Update stdlib for go1.13 (#351)

* chore: generate stdlib for go1.13

* chore: update CI configuration.

* fix: struct13 test is too dependent of the Go implementation.

* fix: disable test import4.go

stdlib/unsafe/go1_13_unsafe.go

@@ -0,0 +1,17 @@
+// Code generated by 'goexports unsafe'. DO NOT EDIT.
+
+// +build go1.13,!go1.14
+
+package unsafe
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+func init() {
+	Symbols["unsafe"] = map[string]reflect.Value{
+		// type definitions
+		"Pointer": reflect.ValueOf((*unsafe.Pointer)(nil)),
+	}
+}

stdlib/unsafe/unsafe.go

@@ -1,4 +1,4 @@
-// +build go1.11,!go1.13
+// +build go1.11,!go1.14
 
 package unsafe