Move src/mobile into main repository (#864)

* Move `src/mobile` into main repository

* Update go.mod/go.sum

* Move to `contrib`, separate mobile build script

contrib/mobile/mobile_other.go

@@ -0,0 +1,13 @@
+// +build !android,!ios
+
+package mobile
+
+import "fmt"
+
+type MobileLogger struct {
+}
+
+func (nsl MobileLogger) Write(p []byte) (n int, err error) {
+	fmt.Print(string(p))
+	return len(p), nil
+}