Enable serde before dependencies

In the `use-serde` feature we enable `serde` and also enable it in our
dependencies. Put `serde` first in the list under the philosophy of
'most important thing first'. This is only subjectively better.

Author: heap-coder

Cargo.toml

@@ -14,7 +14,7 @@ compiler = []
 trace = []
 unstable = []
 default = []
-use-serde = ["bitcoin/use-serde", "serde"]
+use-serde = ["serde", "bitcoin/use-serde"]
 rand = ["bitcoin/rand"]
 
 [dependencies]