Using serde/alloc and serde/std with corresponding features

Author: dr-orlovsky

Cargo.toml

@@ -19,9 +19,9 @@ features = [ "rand", "rand-std", "serde", "recovery" ]
 [features]
 unstable = ["recovery", "rand-std"]
 default = ["std"]
-std = ["secp256k1-sys/std"]
+std = ["secp256k1-sys/std", "serde/std"]
 # allow use of Secp256k1::new and related API that requires an allocator
-alloc = []
+alloc = ["serde/alloc"]
 rand-std = ["rand/std"]
 recovery = ["secp256k1-sys/recovery"]
 lowmemory = ["secp256k1-sys/lowmemory"]