Transition build-manifest to 2018 edition

Author: h-michael

src/tools/build-manifest/Cargo.toml

@@ -2,6 +2,7 @@
 name = "build-manifest"
 version = "0.1.0"
 authors = ["Alex Crichton <[email protected]>"]
+edition = "2018"
 
 [dependencies]
 toml = "0.4"

src/tools/build-manifest/src/main.rs

@@ -1,4 +1,6 @@
-extern crate toml;
+#![deny(rust_2018_idioms)]
+
+use toml;
 #[macro_use]
 extern crate serde_derive;