sim: simflash: Minimal 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/simflash/src/lib.rs b/sim/simflash/src/lib.rs
index df1020e..26f8187 100644
--- a/sim/simflash/src/lib.rs
+++ b/sim/simflash/src/lib.rs
@@ -15,7 +15,7 @@
 use std::path::Path;
 use std::slice;
 use std::collections::HashMap;
-use pdump::HexDump;
+use crate::pdump::HexDump;
 
 error_chain! {
     errors {