Change object representation from 1 big union to individual structs.

A big change.  Micro Python objects are allocated as individual structs
with the first element being a pointer to the type information (which
is itself an object).  This scheme follows CPython.  Much more flexible,
not necessarily slower, uses same heap memory, and can allocate objects
statically.

Also change name prefix, from py_ to mp_ (mp for Micro Python).

py/objclosure.c

@@ -0,0 +1,35 @@
+#include <stdlib.h>
+#include <stdint.h>
+#include <assert.h>
+
+#include "nlr.h"
+#include "misc.h"
+#include "mpconfig.h"
+#include "obj.h"
+#include "runtime.h"
+
+typedef struct _mp_obj_closure_t {
+    mp_obj_base_t base;
+    mp_obj_t fun;
+    mp_obj_t vars;
+} mp_obj_closure_t;
+
+const mp_obj_type_t closure_type = {
+    { &mp_const_type },
+    "closure",
+    NULL, // print
+    NULL, // call_n
+    NULL, // unary_op
+    NULL, // binary_op
+    NULL, // getiter
+    NULL, // iternext
+    {{NULL, NULL},}, // method list
+};
+
+mp_obj_t mp_obj_new_closure(mp_obj_t fun, mp_obj_t closure_tuple) {
+    mp_obj_closure_t *o = m_new_obj(mp_obj_closure_t);
+    o->base.type = &closure_type;
+    o->fun = fun;
+    o->vars = closure_tuple;
+    return o;
+}