derive macro silently fails when ran on a tuple struct

Nobody has claimed this yet.

Apologies, this isn't exactly a minimal test, just how I discovered it.

```rust
use binread::{BinRead, NullString};
use binwrite::BinWrite;

// this *compiles* but does not behave as exepected.
#[derive(BinRead, BinWrite, Clone, PartialEq, Default)]
pub struct BadNullString(
#[binwrite(cstr, preprocessor(NullString::to_string))]
NullString
);

#[test]
fn test_bad() {
let test = BadNullString(NullString(Vec::from(&b"Test!"[..])));

let expected = Vec::from(&b"Test!\0"[..]);

let mut writer = std::io::Cursor::new(Vec::new());
test.write(&mut writer).unwrap();

let res = writer.into_inner();
assert_eq!(res, expected); // fails, res has no bytes in it
}

// this works correctly
#[derive(BinRead, BinWrite, Clone, PartialEq, Default)]
pub struct GoodNullString {
#[binwrite(cstr, preprocessor(NullString::to_string))]
inner: NullString
}

#[test]
fn test_good() {
let test = GoodNullString { inner: NullString(Vec::from(&b"Test!"[..])) };

let expected = Vec::from(&b"Test!\0"[..]);

let mut writer = std::io::Cursor::new(Vec::new());
test.write(&mut writer).unwrap();

let res = writer.into_inner();
assert_eq!(res, expected); // passes
}
```

Start by running the provided tuple-struct and named-struct tests, then locate the BinWrite derive macro entry point and compare how each struct shape is expanded. Done means the tuple-struct case writes the expected Test!\0 bytes while the named-struct case continues to pass.

Written by the indexing model from the issue text.