sim: Migrate to Rust 2021 edition

Change the edition in the various Cargo.toml to 2021.  There are no
changes to the code needed for this.  This will require Rust 1.56 in
order to build.
Signed-off-by: David Brown <david.brown@linaro.org>

sim/Cargo.toml

@@ -2,7 +2,7 @@
 name = "bootsim"
 version = "0.1.0"
 authors = ["David Brown <davidb@davidb.org>"]
-edition = "2018"
+edition = "2021"
 
 [features]
 default = []

sim/mcuboot-sys/Cargo.toml

@@ -5,7 +5,7 @@ authors = ["David Brown <david.brown@linaro.org>"]
 description = "A simple wrapper around the mcuboot code."
 build = "build.rs"
 publish = false
-edition = "2018"
+edition = "2021"
 
 [features]
 # By default, build with simplistic signature verification.

sim/simflash/Cargo.toml

@@ -2,7 +2,7 @@
 name = "simflash"
 version = "0.1.0"
 authors = ["David Brown <david.brown@linaro.org>"]
-edition = "2018"
+edition = "2021"
 
 [dependencies]
 rand = "0.7"