serde support

[jymchng]

(in pseudocode)

#![cfg(feature = "serializable-secret")]

...

Secret<T: SerializableSecret, MEC, EC>.serialize_secret<EC: EncryptionKeyTrait>(ec: EC) -> SerializableSecret<T: SerializableSecret, MEC, EC>

Then impl serde::Serialize for SerializableSecret<_, _, _>.

The advantages are:

1. Feature-gated.
2. Even if feature is enabled, user of the library can choose to only impl SerializableSecret for some-of-their-T (library will impl for common Ts in std).