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/api.rs b/sim/src/api.rs
index 2fa6b31..0347b65 100644
--- a/sim/src/api.rs
+++ b/sim/src/api.rs
@@ -1,6 +1,6 @@
 //! HAL api for MyNewt applications
 
-use flash::{Result, Flash};
+use simflash::{Result, Flash};
 use libc;
 use log::LogLevel;
 use mem;