Implementations of persistent-code reader are provided for POSIX systems
and systems using FatFS. Macros to use these are MICROPY_READER_POSIX and
MICROPY_READER_FATFS respectively. If an alternative implementation is
needed then a port can define the function mp_reader_new_file.
This type was used only for the typedef of mp_obj_t, which is now defined
by the object representation. So we can now remove this unused typedef,
to simplify the mpconfigport.h file.
Damien George