py: Factor out persistent-code reader into separate files.

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.

py/persistentcode.h

@@ -26,13 +26,9 @@
 #ifndef MICROPY_INCLUDED_PY_PERSISTENTCODE_H
 #define MICROPY_INCLUDED_PY_PERSISTENTCODE_H
 
-#include "py/obj.h"
-
-typedef struct _mp_reader_t {
-    void *data;
-    mp_uint_t (*read_byte)(void *data);
-    void (*close)(void *data);
-} mp_reader_t;
+#include "py/mpprint.h"
+#include "py/reader.h"
+#include "py/emitglue.h"
 
 mp_raw_code_t *mp_raw_code_load(mp_reader_t *reader);
 mp_raw_code_t *mp_raw_code_load_mem(const byte *buf, size_t len);