py/stream: Implement 2- and 3-arg write() method as an extension to CPython.

3-arg form:

stream.write(data, offset, length)

2-arg form:

stream.write(data, length)

These allow efficient buffer writing without incurring extra memory
allocation for slicing or creating memoryview() object, what is
important for low-memory ports.

All arguments must be positional. It might be not so bad idea to standardize
on 3-arg form, but 2-arg case would need check and raising an exception
anyway then, so instead it was just made to work.

py/stream.c

@@ -267,12 +267,24 @@ void mp_stream_write_adaptor(void *self, const char *buf, size_t len) {
     mp_stream_write(MP_OBJ_FROM_PTR(self), buf, len, MP_STREAM_RW_WRITE);
 }
 
-STATIC mp_obj_t stream_write_method(mp_obj_t self_in, mp_obj_t arg) {
+STATIC mp_obj_t stream_write_method(size_t n_args, const mp_obj_t *args) {
     mp_buffer_info_t bufinfo;
-    mp_get_buffer_raise(arg, &bufinfo, MP_BUFFER_READ);
-    return mp_stream_write(self_in, bufinfo.buf, bufinfo.len, MP_STREAM_RW_WRITE);
+    mp_get_buffer_raise(args[1], &bufinfo, MP_BUFFER_READ);
+    size_t max_len = (size_t)-1;
+    size_t off = 0;
+    if (n_args == 3) {
+        max_len = mp_obj_get_int_truncated(args[2]);
+    } else if (n_args == 4) {
+        off = mp_obj_get_int_truncated(args[2]);
+        max_len = mp_obj_get_int_truncated(args[3]);
+        if (off > bufinfo.len) {
+            off = bufinfo.len;
+        }
+    }
+    bufinfo.len -= off;
+    return mp_stream_write(args[0], (byte*)bufinfo.buf + off, MIN(bufinfo.len, max_len), MP_STREAM_RW_WRITE);
 }
-MP_DEFINE_CONST_FUN_OBJ_2(mp_stream_write_obj, stream_write_method);
+MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(mp_stream_write_obj, 2, 4, stream_write_method);
 
 STATIC mp_obj_t stream_write1_method(mp_obj_t self_in, mp_obj_t arg) {
     mp_buffer_info_t bufinfo;