Add __rust_alloc_zeroed to naive_ralloc

This is fallout from rust-lang/rust#40409 which requires that all
allocators provide a `__rust_alloc_zeroed` function.

Fixes japaric#136.

naive_ralloc/src/lib.rs

@@ -36,6 +36,16 @@ pub extern fn __rust_allocate(size: usize, _align: usize) -> *mut u8 {
     allocate(size)
 }
 
+#[linkage = "external"]
+#[no_mangle]
+pub extern fn __rust_allocate_zeroed(size: usize, align: usize) -> *mut u8 {
+    unsafe {
+        let result = __rust_allocate(size, align);
+        intrinsics::write_bytes(result, 0, size);
+        result
+    }
+}
+
 #[linkage = "external"]
 #[no_mangle]
 pub extern fn __rust_deallocate(_ptr: *mut u8, _old_size: usize, _align: usize) {