py/modsys: Report .mpy version in sys.implementation.

This commit adds a sys.implementation.mpy entry when the system supports
importing .mpy files.  This entry is a 16-bit integer which encodes two
bytes of information from the header of .mpy files that are supported by
the system being run: the second and third bytes, .mpy version, and flags
and native architecture.  This allows determining the supported .mpy file
dynamically by code, and also for the user to find it out by inspecting
this value.  It's further possible to dynamically detect if the system
supports importing .mpy files by `hasattr(sys.implementation, 'mpy')`.

py/modsys.c

@@ -34,6 +34,7 @@
 #include "py/stream.h"
 #include "py/smallint.h"
 #include "py/runtime.h"
+#include "py/persistentcode.h"
 
 #if MICROPY_PY_SYS_SETTRACE
 #include "py/objmodule.h"
@@ -66,22 +67,36 @@ STATIC const mp_obj_tuple_t mp_sys_implementation_version_info_obj = {
     3,
     { I(MICROPY_VERSION_MAJOR), I(MICROPY_VERSION_MINOR), I(MICROPY_VERSION_MICRO) }
 };
+#if MICROPY_PERSISTENT_CODE_LOAD
+#define SYS_IMPLEMENTATION_ELEMS \
+    MP_ROM_QSTR(MP_QSTR_micropython), \
+    MP_ROM_PTR(&mp_sys_implementation_version_info_obj), \
+    MP_ROM_INT(MPY_FILE_HEADER_INT)
+#else
+#define SYS_IMPLEMENTATION_ELEMS \
+    MP_ROM_QSTR(MP_QSTR_micropython), \
+    MP_ROM_PTR(&mp_sys_implementation_version_info_obj)
+#endif
 #if MICROPY_PY_ATTRTUPLE
-STATIC const qstr impl_fields[] = { MP_QSTR_name, MP_QSTR_version };
+STATIC const qstr impl_fields[] = {
+    MP_QSTR_name,
+    MP_QSTR_version,
+    #if MICROPY_PERSISTENT_CODE_LOAD
+    MP_QSTR_mpy,
+    #endif
+};
 STATIC MP_DEFINE_ATTRTUPLE(
     mp_sys_implementation_obj,
     impl_fields,
-    2,
-        MP_ROM_QSTR(MP_QSTR_micropython),
-        MP_ROM_PTR(&mp_sys_implementation_version_info_obj)
+    2 + MICROPY_PERSISTENT_CODE_LOAD,
+        SYS_IMPLEMENTATION_ELEMS
 );
 #else
 STATIC const mp_rom_obj_tuple_t mp_sys_implementation_obj = {
     {&mp_type_tuple},
-    2,
+    2 + MICROPY_PERSISTENT_CODE_LOAD,
     {
-        MP_ROM_QSTR(MP_QSTR_micropython),
-        MP_ROM_PTR(&mp_sys_implementation_version_info_obj),
+        SYS_IMPLEMENTATION_ELEMS
     }
 };
 #endif