Conform to Cargo's conventional file layout

Move `src/*.rs` to `src/bin/*.rs` which are automatically inferred as
binaries and move `src/utils.rs` to `src/lib.rs` which is compiled as a
reusable library for each of the binaries we're building.

src/lib.rs

@@ -0,0 +1,25 @@
+pub fn init_file_per_thread_logger(prefix: &'static str) {
+    file_per_thread_logger::initialize(prefix);
+
+    // Extending behavior of default spawner:
+    // https://docs.rs/rayon/1.1.0/rayon/struct.ThreadPoolBuilder.html#method.spawn_handler
+    // Source code says DefaultSpawner is implementation detail and
+    // shouldn't be used directly.
+    rayon::ThreadPoolBuilder::new()
+        .spawn_handler(move |thread| {
+            let mut b = std::thread::Builder::new();
+            if let Some(name) = thread.name() {
+                b = b.name(name.to_owned());
+            }
+            if let Some(stack_size) = thread.stack_size() {
+                b = b.stack_size(stack_size);
+            }
+            b.spawn(move || {
+                file_per_thread_logger::initialize(prefix);
+                thread.run()
+            })?;
+            Ok(())
+        })
+        .build_global()
+        .unwrap();
+}