Specify edition to 2018 in Cargo.toml

Cargo.toml

@@ -6,6 +6,7 @@ repository = "https://github.com/rust-osdev/acpi"
 description = "Library for parsing ACPI tables and AML"
 readme = "README.md"
 license = "MIT/Apache-2.0"
+edition = "2018"
 
 [dependencies]
 log = "0.*"