py, readline: Add tab autocompletion for REPL.

Can complete names in the global namespace, as well as a chain of
attributes, eg pyb.Pin.board.<tab> will give a list of all board pins.

Costs 700 bytes ROM on Thumb2 arch, but greatly increases usability of
REPL prompt.

py/repl.h

@@ -28,9 +28,11 @@
 
 #include "py/mpconfig.h"
 #include "py/misc.h"
+#include "py/mpprint.h"
 
 #if MICROPY_HELPER_REPL
 bool mp_repl_continue_with_input(const char *input);
+mp_uint_t mp_repl_autocomplete(const char *str, mp_uint_t len, const mp_print_t *print, const char **compl_str);
 #endif
 
 #endif // __MICROPY_INCLUDED_PY_REPL_H__