Introduce feature-gated `serde` dependency

* derive `serde::{Deserialize, Serialize}` for `Capability` and `AuthMechanism`.

Cargo.toml

@@ -10,6 +10,13 @@ edition = "2018"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[features]
+default = []
+serdex  = ["serde"]
+
 [dependencies]
 nom = "6"
 abnf-core = "0.4.0"
+
+# Optional
+serde = { version = "1.0", features = ["derive"], optional = true }

src/types.rs

@@ -1,4 +1,6 @@
 use crate::utils::escape_quoted;
+#[cfg(feature = "serdex")]
+use serde::{Deserialize, Serialize};
 use std::io::Write;
 
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -321,6 +323,7 @@ impl EhloOkResp {
 
 // -------------------------------------------------------------------------------------------------
 
+#[cfg_attr(feature = "serdex", derive(Serialize, Deserialize))]
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum Capability {
     // Send as mail [RFC821]
@@ -502,6 +505,7 @@ impl Capability {
     }
 }
 
+#[cfg_attr(feature = "serdex", derive(Serialize, Deserialize))]
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum AuthMechanism {
     Plain,