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/builtin.h b/py/builtin.h
index 1bb61f6..9c8b2b9 100644
--- a/py/builtin.h
+++ b/py/builtin.h
@@ -89,3 +89,4 @@
 // extmod modules
 extern const mp_obj_module_t mp_module_uctypes;
 extern const mp_obj_module_t mp_module_zlibd;
+extern const mp_obj_module_t mp_module_ujson;