py: Add native json printing using existing print framework.

Also add start of ujson module with dumps implemented.  Enabled in unix
and stmhal ports.  Test passes on both.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index adbcb0e..cf85533 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -390,6 +390,10 @@
 #define MICROPY_PY_ZLIBD (0)
 #endif
 
+#ifndef MICROPY_PY_UJSON
+#define MICROPY_PY_UJSON (0)
+#endif
+
 /*****************************************************************************/
 /* Hooks for a port to add builtins                                          */