Cargo.toml: Make "collections" feature optional

Cargo.toml

@@ -14,7 +14,12 @@ description = "Support for bumpalo in scoped threads & rayon"
 bumpalo = "~3"
 
 [dev-dependencies]
-bumpalo = { version = "~3", features = ["collections"] }
+bumpalo = "~3"
 crossbeam-utils = "~0.8"
 num_cpus = "~1"
 rayon = "~1"
+
+[features]
+default = [
+  "bumpalo/collections", # Exists for backward compatibility. Please remove this line at 0.2.0 release.
+]