sim: Move `areadesc` into `Images` struct

Move this struct into the images struct as well to avoid passing it
around with every call.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/simflash/src/lib.rs b/sim/simflash/src/lib.rs
index 5683595..300802c 100644
--- a/sim/simflash/src/lib.rs
+++ b/sim/simflash/src/lib.rs
@@ -184,7 +184,7 @@
 }
 
 /// It is possible to iterate over the sectors in the device, each element returning this.
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct Sector {
     /// Which sector is this, starting from 0.
     pub num: usize,