print walltime benchmarks with subnanosecond precision

example results when benchmarking 1-4 serialized ADD instructions

```
running 4 tests
test add  ... bench:           0.24 ns/iter (+/- 0.00)
test add2 ... bench:           0.48 ns/iter (+/- 0.01)
test add3 ... bench:           0.72 ns/iter (+/- 0.01)
test add4 ... bench:           0.96 ns/iter (+/- 0.01)
```

Author: the8472

test/src/bench.rs

@@ -68,12 +68,12 @@ pub fn fmt_bench_samples(bs: &BenchSamples) -> String {
     use std::fmt::Write;
     let mut output = String::new();
 
-    let median = bs.ns_iter_summ.median as usize;
-    let deviation = (bs.ns_iter_summ.max - bs.ns_iter_summ.min) as usize;
+    let median = bs.ns_iter_summ.median;
+    let deviation = bs.ns_iter_summ.max - bs.ns_iter_summ.min;
 
     write!(
         output,
-        "{:>11} ns/iter (+/- {})",
+        "{:>14} ns/iter (+/- {})",
         fmt_thousands_sep(median, ','),
         fmt_thousands_sep(deviation, ',')
     )
@@ -85,24 +85,27 @@ pub fn fmt_bench_samples(bs: &BenchSamples) -> String {
 }
 
 // Format a number with thousands separators
-fn fmt_thousands_sep(mut n: usize, sep: char) -> String {
+fn fmt_thousands_sep(mut n: f64, sep: char) -> String {
     use std::fmt::Write;
     let mut output = String::new();
     let mut trailing = false;
     for &pow in &[9, 6, 3, 0] {
         let base = 10_usize.pow(pow);
-        if pow == 0 || trailing || n / base != 0 {
-            if !trailing {
-                write!(output, "{}", n / base).unwrap();
-            } else {
-                write!(output, "{:03}", n / base).unwrap();
+        if pow == 0 || trailing || n / base as f64 >= 1.0 {
+            match (pow, trailing) {
+                // modern CPUs can execute multiple instructions per nanosecond
+                // e.g. benching an ADD takes about 0.25ns.
+                (0, true) => write!(output, "{:06.2}", n / base as f64).unwrap(),
+                (0, false) => write!(output, "{:.2}", n / base as f64).unwrap(),
+                (_, true) => write!(output, "{:03}", n as usize / base).unwrap(),
+                _ => write!(output,  "{}", n as usize / base).unwrap()
             }
             if pow != 0 {
                 output.push(sep);
             }
             trailing = true;
         }
-        n %= base;
+        n %= base as f64;
     }
 
     output