py: Change type signature of builtin funs that take variable or kw args.

With this patch the n_args parameter is changed type from mp_uint_t to
size_t.
diff --git a/py/stream.c b/py/stream.c
index d29db62..82bdffe 100644
--- a/py/stream.c
+++ b/py/stream.c
@@ -62,7 +62,7 @@
     return stream_p;
 }
 
-STATIC mp_obj_t stream_read(mp_uint_t n_args, const mp_obj_t *args) {
+STATIC mp_obj_t stream_read(size_t n_args, const mp_obj_t *args) {
     const mp_stream_p_t *stream_p = mp_get_stream_raise(args[0], MP_STREAM_OP_READ);
 
     // What to do if sz < -1?  Python docs don't specify this case.
@@ -214,7 +214,7 @@
     return mp_stream_write(self_in, bufinfo.buf, bufinfo.len);
 }
 
-STATIC mp_obj_t stream_readinto(mp_uint_t n_args, const mp_obj_t *args) {
+STATIC mp_obj_t stream_readinto(size_t n_args, const mp_obj_t *args) {
     const mp_stream_p_t *stream_p = mp_get_stream_raise(args[0], MP_STREAM_OP_READ);
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[1], &bufinfo, MP_BUFFER_WRITE);
@@ -287,7 +287,7 @@
 }
 
 // Unbuffered, inefficient implementation of readline() for raw I/O files.
-STATIC mp_obj_t stream_unbuffered_readline(mp_uint_t n_args, const mp_obj_t *args) {
+STATIC mp_obj_t stream_unbuffered_readline(size_t n_args, const mp_obj_t *args) {
     const mp_stream_p_t *stream_p = mp_get_stream_raise(args[0], MP_STREAM_OP_READ);
 
     mp_int_t max_size = -1;
@@ -365,7 +365,7 @@
     return MP_OBJ_STOP_ITERATION;
 }
 
-STATIC mp_obj_t stream_seek(mp_uint_t n_args, const mp_obj_t *args) {
+STATIC mp_obj_t stream_seek(size_t n_args, const mp_obj_t *args) {
     const mp_stream_p_t *stream_p = mp_get_stream_raise(args[0], MP_STREAM_OP_IOCTL);
 
     struct mp_stream_seek_t seek_s;