Add property-get utility function

This function will look for a given property in a given table. It gives
back either the property, or an explanation of why it could not be
retrieved.

src/config.rs

@@ -1,8 +1,21 @@
+use toml::{Value, value::Table};
 
 pub type Result<T> = core::result::Result<T, Error>;
 
 #[derive(Debug)]
-pub enum Error {}
+#[cfg_attr(test, derive(PartialEq))]
+pub enum Error {
+    BadFormat,
+    NoSuchProperty,
+    NoSuchTable,
+    TomlWrap(toml::de::Error),
+}
+
+impl From<toml::de::Error> for Error {
+    fn from(value: toml::de::Error) -> Self {
+        Error::TomlWrap(value)
+    }
+}
 
 impl core::fmt::Display for Error{
 	fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
@@ -13,7 +26,43 @@ impl core::fmt::Display for Error{
 
 impl std::error::Error for Error {}
 
+/// The config properties are individual strings. This gets the named property,
+/// or an error explaining why it couldn't be fetched.
+fn get_property<'outer>(outer: &'outer Table, property: String) -> Result<&'outer String> {
+    let maybe_prop = outer.get(&property).ok_or(Error::NoSuchProperty)?;
+    if let Value::String(text) = maybe_prop {
+        Ok(text)
+    } else {
+        Err(Error::BadFormat)
+    }
+}
+
 #[cfg(test)]
 mod tests {
-	use super::*;
-}
+    use super::*;
+
+    #[test]
+    fn read_single_prop() -> Result<()> {
+        let fx_input_str = "owner = \"dingus\"";
+        let fx_value = fx_input_str.parse::<Value>()?;
+        let fx_value = fx_value.as_table().ok_or(Error::NoSuchTable)?;
+        let expected = "dingus";
+
+        let res = get_property(&fx_value, String::from("owner"))?;
+        assert_eq!(res, expected);
+        Ok(())
+    }
+
+    // The property is given the value of empty-string `""`
+    #[test]
+    fn read_single_prop_empty_quotes() -> Result<()> {
+        let fx_input_str = "owner = \"\"";
+        let fx_value = fx_input_str.parse::<Value>()?;
+        let fx_value = fx_value.as_table().ok_or(Error::NoSuchTable)?;
+        let expected = "";
+
+        let res = get_property(&fx_value, String::from("owner"))?;
+        assert_eq!(res, expected);
+        Ok(())
+    }
+}