sim: fixes for 2018

Automatic migration to Rust 2018.  This is the result of running

    cargo fix --edition

The resulting code is compatible with both Rust 2015 and 2018.  Change
the edition field in the Cargo.toml file as well to begin a more
complete migration.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/src/lib.rs b/sim/src/lib.rs
index 0fc9363..6cb6f28 100644
--- a/sim/src/lib.rs
+++ b/sim/src/lib.rs
@@ -30,8 +30,8 @@
 
 use simflash::{Flash, SimFlash, SimFlashMap};
 use mcuboot_sys::{c, AreaDesc, FlashId};
-use caps::Caps;
-use tlv::{TlvGen, TlvFlags, AES_SEC_KEY};
+use crate::caps::Caps;
+use crate::tlv::{TlvGen, TlvFlags, AES_SEC_KEY};
 
 const USAGE: &'static str = "
 Mcuboot simulator