config: Add ItemOrBool helper for deserializing a bool or a value.

This will enable us to support the new `keystore_dir` field, which can
be either a boolean indicating whether the keystore is disabled or
enabled and initialized with the default keystore dir, or a string which
points to a custom keystore directory (and implies the keystore is
enabled):
```
# use this path, fail if compiled out
# keystore = "/path/to/arti/keystore"
#
# use default path, fail if compiled out
# keystore = true
#
# disable
# keystore = false
```

Part of #891

crates/tor-config/semver.md

@@ -0,0 +1 @@
+ADDED: `ItemOrBool`

crates/tor-config/src/misc.rs

@@ -383,6 +383,31 @@ impl TryFrom<ListenItemSerde> for ListenItem {
     }
 }
 
+/// A deserializable value or bool.
+#[derive(Serialize, Deserialize, Debug)]
+#[allow(clippy::exhaustive_enums)] // we will add variants very rarely if ever
+#[serde(untagged, bound = "T: Serialize, for<'de2> T: Deserialize<'de2>")]
+pub enum ItemOrBool<T>
+where
+    T: std::fmt::Debug + Serialize,
+    for<'de2> T: Deserialize<'de2>,
+{
+    /// A `T` item.
+    Item(T),
+    /// A boolean value
+    Bool(bool),
+}
+
+impl<T> Default for ItemOrBool<T>
+where
+    T: std::fmt::Debug + Serialize,
+    for<'de2> T: Deserialize<'de2>,
+{
+    fn default() -> Self {
+        Self::Bool(true)
+    }
+}
+
 #[cfg(test)]
 mod test {
     // @@ begin test lint list maintained by maint/add_warning @@
@@ -407,6 +432,9 @@ mod test {
 
         #[serde(default)]
         listen: Option<Listen>,
+
+        #[serde(default)]
+        enabled_or_string: ItemOrBool<String>,
     }
 
     #[test]
@@ -555,4 +583,35 @@ mod test {
         chk_err_1("need actual addr/port", "did not match any variant", "true");
         chk_err("did not match any variant", r#"listen = [ [] ]"#);
     }
+
+    #[test]
+    fn enabled_or_string() {
+        use ItemOrBool as IOB;
+
+        let chk = |iob: IOB<String>, s| {
+            let tc: TestConfigFile = toml::from_str(s).expect(s);
+            assert_eq!(
+                format!("{:?}", iob),
+                format!("{:?}", tc.enabled_or_string),
+                "{:?}",
+                s
+            );
+        };
+
+        chk(IOB::Bool(false), r#"enabled_or_string = false"#);
+        chk(IOB::Bool(true), r#"enabled_or_string = true"#);
+        chk(
+            IOB::Item("not a bool".into()),
+            r#"enabled_or_string = "not a bool""#,
+        );
+
+        let chk_e = |s| {
+            let tc: Result<TestConfigFile, _> = toml::from_str(s);
+            let _ = tc.expect_err(s);
+        };
+
+        chk_e(r#"enabled_or_string = 1"#);
+        chk_e(r#"enabled_or_string = []"#);
+        chk_e(r#"enabled_or_string = {}"#);
+    }
 }