Explicitly bind lifetimes for NlaBuffer.value()

Without this, the following results in an error:

```rust
fn subnlas_by_kind<'a>(nlas: &'a [u8]) -> HashMap<u16, Vec<&'a [u8]>> {
	let mut out = HashMap::new();
	for n in NlasIterator::new(nlas) {
		let n = n.unwrap();
		let entry: &mut Vec<_> = out.entry(n.kind()).or_default();
		(*entry).push(n.value())
	}
	out
}
```

```
error[E0515]: cannot return value referencing local variable `n`
    |
212 |         (*entry).push(n.value())
    |                       - `n` is borrowed here
213 |     }
214 |     out
    |     ^^^ returns a value referencing data owned by the current function
```

Author: vthriller

src/nla.rs

@@ -148,9 +148,9 @@ impl<T: AsRef<[u8]> + AsMut<[u8]>> NlaBuffer<T> {
     }
 }
 
-impl<T: AsRef<[u8]> + ?Sized> NlaBuffer<&T> {
+impl<'buf: 'val, 'val, T: AsRef<[u8]> + ?Sized> NlaBuffer<&'buf T> {
     /// Return the `value` field
-    pub fn value(&self) -> &[u8] {
+    pub fn value(&self) -> &'val [u8] {
         &self.buffer.as_ref()[VALUE(self.value_length())]
     }
 }