Derive Debug for types where not all fields implement Debug
```rust
use partialdebug::non_exhaustive::PartialDebug;
struct Dog { legs: usize, eyes: usize, dna: DNA, }
assert_eq!(format!("{:?}", Dog::new()), "Dog { legs: 4, eyes: 2, .. }"); ```
```rust use partialdebug::placeholder::PartialDebug;
struct Dog { legs: usize, eyes: usize, dna: DNA, }
assert_eq!(format!("{:?}", Dog::new()), "Dog { legs: 4, eyes: 2, dna: DNA }"); ```
```rust use partialdebug::placeholder::PartialDebug;
struct Dog { legs: usize, eyes: usize, dna: DNA, }
assert_eq!(format!("{:?}", Dog::new()), "Dog { legs: 4, eyes: 2, dna: Unknown }"); ```
Licensed under either of Apache License, Version 2.0 or MIT license at your option.
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