Update rand to 0.8.3

This update in particular changes `SmallRng` to xoshiro. `SmallRng` is
only used for scene construction, as the core continues to use
`thread_rng`, which wraps `StdRng`. See
rust-random/rand#1038 for more.

As expected, benchmarking shows that this change is performance-neutral
and the output looks correct.

There is an ongoing experimental branch that effectively implements a
custom thread-local RNG solution using a custom rayon pool. This will
in particular allow using xoshiro in the core trace loop.

Benchmarking:

AMD Ryzen 9 3900X 12-Core Processor (AMD64 Family 23 Model 113 Stepping 0)

tracescene/10x10x4      time:   [295.42 us 295.79 us 296.17 us]
                        change: [-1.4129% -1.1793% -0.9318%] (p = 0.00 < 0.05)
                        Change within noise threshold.
Found 8 outliers among 100 measurements (8.00%)
  1 (1.00%) low severe
  5 (5.00%) low mild
  1 (1.00%) high mild
  1 (1.00%) high severe

Cargo.toml

@@ -7,7 +7,7 @@ edition = "2018"
 [dependencies]
 image = "0.23"
 palette = "0.5"
-rand = "0.7"
+rand = { version = "0.8.3", features = ["small_rng"] }
 rayon = "1.5.0"
 vek = "0.14.0"