chore: add alloc feature for future compat

Author: kchibisov

Cargo.toml

@@ -16,4 +16,5 @@ serde = { version = "1.0.162", default-features = false, features = ["derive"],
 
 [features]
 default = ["std"]
-std = []
+std = ["alloc"]
+alloc = []

src/lib.rs

@@ -85,6 +85,10 @@
 #[macro_use]
 extern crate serde;
 
+// XXX for forwards compatibility.
+#[cfg(feature = "alloc")]
+extern crate alloc as _;
+
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 ///
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