branch out the RNG

this is basically what !130 is trying to do.

rng-branching/src/main.rs

 use clap::Parser;
 use rand::{rngs::StdRng, Rng, RngCore, SeedableRng};
 
-fn measure(nb_measurements: usize, rng: &mut impl RngCore) {
+fn measure(nb_measurements: usize, rng: &(impl RngCore + 'static)) {
+    let mut rng: StdRng = (rng as &dyn std::any::Any)
+        .downcast_ref::<StdRng>()
+        .unwrap()
+        .clone();
+
     for _ in 0..nb_measurements {
         println!("    {}", rng.gen::<u128>());
     }
@@ -11,7 +16,7 @@ fn run<Fun: Fn(usize) -> bool>(
     max_t: usize,
     nb_measurements: usize,
     measurement_schedule: Fun,
-    rng: &mut impl RngCore,
+    rng: &mut (impl RngCore + 'static),
 ) {
     println!("0: {}", rng.gen::<u128>());
     for t in 1..=max_t {