Add repr(C) to i256 (#5837)

Author: tustvold

arrow-buffer/src/bigint/mod.rs

@@ -54,6 +54,7 @@ enum DivRemError {
 /// A signed 256-bit integer
 #[allow(non_camel_case_types)]
 #[derive(Copy, Clone, Default, Eq, PartialEq, Hash)]
+#[repr(C)]
 pub struct i256 {
     low: u128,
     high: i128,