py/stream: Add mp_stream_writeall() helper function.

Spools entire output buffer to a blocking stream (chunk by chunk if
needed).

py/stream.h

@@ -49,6 +49,9 @@ mp_obj_t mp_stream_unbuffered_iter(mp_obj_t self);
 
 mp_obj_t mp_stream_write(mp_obj_t self_in, const void *buf, size_t len);
 
+// Helper function to write entire buf to *blocking* stream
+mp_uint_t mp_stream_writeall(mp_obj_t stream, const byte *buf, mp_uint_t size, int *errcode);
+
 #if MICROPY_STREAMS_NON_BLOCK
 // TODO: This is POSIX-specific (but then POSIX is the only real thing,
 // and anything else just emulates it, right?)