sim: Move simflash to be its own crate

In preparation for moving the bootutil binding into a separate crate,
move the flash simulator into its own crate.  This will allow the
binding to have access to the simulator without creating a circular
dependency.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/src/area.rs b/sim/src/area.rs
index eb42292..5a1ef6c 100644
--- a/sim/src/area.rs
+++ b/sim/src/area.rs
@@ -1,6 +1,6 @@
 //! Describe flash areas.
 
-use flash::{Flash, SimFlash, Sector};
+use simflash::{Flash, SimFlash, Sector};
 use std::ptr;
 
 /// Structure to build up the boot area table.