feat: simplify getrandom call (#325)

New in getrandom 0.3, we can now get numbers directly. On some systems,
this'll use special CPU instructions and can be instantaneous. We're not
using them for crypto, so I'm not going to be picky.

src/util.rs

@@ -58,9 +58,8 @@ pub fn create_helper<R>(
             any(windows, unix, target_os = "redox", target_os = "wasi")
         ))]
         if i == 3 {
-            let mut seed = [0u8; 8];
-            if getrandom::fill(&mut seed).is_ok() {
-                rng.seed(u64::from_ne_bytes(seed));
+            if let Ok(seed) = getrandom::u64() {
+                rng.seed(seed);
             }
         }
         let path = base.join(tmpname(&mut rng, prefix, suffix, random_len));