feat: add as type casting

tests/cast_int_to_int.test

@@ -0,0 +1,23 @@
+[code]
+fn truncate_i32_to_i8(x: i32) -> i8 {
+    return x as i8;
+}
+
+fn extend_u8_to_i32(x: u8) -> i32 {
+    return x as i32;
+}
+
+[harness]
+#include <stdint.h>
+
+extern int8_t truncate_i32_to_i8(int32_t x);
+extern int32_t extend_u8_to_i32(uint8_t x);
+
+int main() {
+    if (truncate_i32_to_i8(257) != 1) return 1; // 257 = 0x0101, truncated to 8 bits is 0x01
+    if (extend_u8_to_i32(255) != 255) return 2;
+    return 0;
+}
+
+[expected_return_code]
+0