Long compile times for structs with large arrays

Using a large array in a struct causes compile times to explode:
```rust
use packed_struct::prelude::*;

#[derive(PackedStruct)]
#[packed_struct(endian = "msb")]
pub struct MyStruct {
    x: u16,
    y: u16,
    d: [u16; 20000],
}

fn main() {}
```
I've tried analyzing with `cargo b --timings` but that didn't yield anything interesting. Let me know if I can help investigate.