Update to Edition 2021. (#359)

smallvec v2's MSRV is already 1.57.0, which is after the relase of Edition 2021 in 1.56.0.

Apply fix for Cargo.toml dev_dependencies warning given by cargo fix --edition.

Author: zachs18

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "smallvec"
 version = "2.0.0-alpha.7"
-edition = "2018"
+edition = "2021"
 rust-version = "1.57"
 authors = ["The Servo Project Developers"]
 license = "MIT OR Apache-2.0"
@@ -21,7 +21,7 @@ extract_if = []
 [dependencies]
 serde = { version = "1", optional = true, default-features = false }
 
-[dev_dependencies]
+[dev-dependencies]
 bincode = "1.0.1"
 
 [package.metadata.docs.rs]

fuzz/Cargo.toml

@@ -2,7 +2,7 @@
 name = "smallvec-fuzz"
 version = "0.1.0"
 authors = ["Dawid Ciężarkiewicz <dpc@dpc.pw>"]
-edition = "2018"
+edition = "2021"
 publish = false
 
 [package.metadata]