refactor: migrate to the 2021 edition

Author: yvt

Cargo.toml

@@ -2,7 +2,6 @@
 members = [
     "crates/*",
 ]
-resolver = "2"
 
 [profile.release]
 debug = true

crates/rlsf/Cargo.toml

@@ -3,7 +3,7 @@ name = "rlsf"
 version = "0.1.2"
 authors = ["yvt <i@yvt.jp>"]
 license = "MIT/Apache-2.0"
-edition = "2018"
+edition = "2021"
 rust-version = "1.61"
 readme = "README.md"
 description = "Real-time dynamic memory allocator based on the TLSF algorithm"