seq: Directly write separator string, instead of using format

Doing `stdout.write_all(separator.as_bytes())?` is quite a bit
faster than using format to do the same operation:
`write!(stdout, "{separator}")?`.

This speeds up by about 10% on simple cases.

We do the same for the terminator even though this has no measurable
performance impact.

src/uu/seq/BENCHMARKING.md

@@ -63,4 +63,18 @@ of system time). Simply wrapping `stdout` in a `BufWriter` increased performance
 by about 2 times for a floating point increment test case, leading to similar
 performance compared with GNU `seq`.
 
+### Directly print strings
+
+As expected, directly printing a string:
+```rust
+stdout.write_all(separator.as_bytes())?
+```
+is quite a bit faster than using format to do the same operation:
+```rust
+write!(stdout, "{separator}")?
+```
+
+The change above resulted in a ~10% speedup.
+
+
 [0]: https://github.com/sharkdp/hyperfine

src/uu/seq/src/seq.rs

@@ -260,7 +260,7 @@ fn print_seq(
     let mut is_first_iteration = true;
     while !done_printing(&value, &increment, &last) {
         if !is_first_iteration {
-            write!(stdout, "{separator}")?;
+            stdout.write_all(separator.as_bytes())?;
         }
         format.fmt(&mut stdout, &value)?;
         // TODO Implement augmenting addition.
@@ -268,7 +268,7 @@ fn print_seq(
         is_first_iteration = false;
     }
     if !is_first_iteration {
-        write!(stdout, "{terminator}")?;
+        stdout.write_all(terminator.as_bytes())?;
     }
     stdout.flush()?;
     Ok(())