py: Add load_const_obj to emitter, add LOAD_CONST_OBJ to bytecode.

This allows to directly load a Python object to the Python stack.  See
issue #722 for background.
diff --git a/py/emitnative.c b/py/emitnative.c
index 494f890..211d289 100644
--- a/py/emitnative.c
+++ b/py/emitnative.c
@@ -1172,6 +1172,12 @@
     }
 }
 
+STATIC void emit_native_load_const_obj(emit_t *emit, void *obj) {
+    emit_native_pre(emit);
+    ASM_MOV_ALIGNED_IMM_TO_REG(emit->as, (mp_uint_t)obj, REG_RET);
+    emit_post_push_reg(emit, VTYPE_PYOBJ, REG_RET);
+}
+
 STATIC void emit_native_load_null(emit_t *emit) {
     emit_native_pre(emit);
     emit_post_push_imm(emit, VTYPE_PYOBJ, 0);
@@ -2274,6 +2280,7 @@
     emit_native_load_const_int,
     emit_native_load_const_dec,
     emit_native_load_const_str,
+    emit_native_load_const_obj,
     emit_native_load_null,
     emit_native_load_fast,
     emit_native_load_deref,