sim: Idiomize for Rust 2018

Apply the changes suggested by

    cargo fix --edition-idioms

as well as a bit of cleanup of the results.  The result should be more
idiomatic Rust 2018 and a good starting point moving forward.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/src/main.rs b/sim/src/main.rs
index 2ced960..2df56d9 100644
--- a/sim/src/main.rs
+++ b/sim/src/main.rs
@@ -1,6 +1,6 @@
-extern crate env_logger;
+use env_logger;
 
-extern crate bootsim;
+use bootsim;
 
 fn main() {
     env_logger::init().unwrap();