clippy: prefering repeat_n instead of repeat

```
error: this `repeat().take()` can be written more concisely
   --> lightning-invoice/src/ser.rs:256:12
    |
256 |         Box::new(core::iter::repeat(Fe32::Q).take(to_pad).chain(fes))
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `repeat_n()` instead: `core::iter::repeat_n(Fe32::Q, to_pad)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_repeat_n
    = note: `-D clippy::manual-repeat-n` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::manual_repeat_n)]`
```

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Author: vincenzopalazzo

lightning-invoice/src/ser.rs

@@ -253,7 +253,7 @@ impl Base32Iterable for PositiveTimestamp {
 		let fes = encode_int_be_base32(self.as_unix_timestamp());
 		debug_assert!(fes.len() <= 7, "Invalid timestamp length");
 		let to_pad = 7 - fes.len();
-		Box::new(core::iter::repeat(Fe32::Q).take(to_pad).chain(fes))
+		Box::new(core::iter::repeat_n(Fe32::Q, to_pad).chain(fes))
 	}
 }