Remove ValueWithKey struct

While using this library, I end up having lifetime issues with
`Config::get`. I've seen that current implementation forces the calleer
to match `key` lifetime to the output of the function.

My use case is, under some circumstances, return a suffixed version of
the config key. Something similar to:

```
        if some_condition == true {
            let key_name = format!("{}_suffix", key);
            self.config.get(&key_name)
        } else {
	    self.config.get(key)
	}
```

This code is noy compiling for me due to conflicting lifetimes. To avoid
this, I've started looking to the code and  I've found that `key` needed
this lifetime because of `ValueWithKey`. The purpouse of this struct
seems to be add more information to the errors that are returned to the
user.

To mitigate this lifetime coupling I've:

- Mapped the error on `Config::get` to include the originating key of
  the current error
- Remove all the code related with `ValueWithKey`

1 files changed, 4 insertions, 3 deletions

diff --git a/src/config.rs b/src/config.rs
index 2ce73f1..7e031d0 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -10,7 +10,7 @@ use ser::ConfigSerializer;
 use source::Source;
 
 use path;
-use value::{Table, Value, ValueKind, ValueWithKey};
+use value::{Table, Value, ValueKind};
 
 #[derive(Clone, Debug)]
 enum ConfigKind {
@@ -151,7 +151,7 @@ impl Config {
         self.refresh()
     }
 
-    pub fn get<'de, T: Deserialize<'de>>(&self, key: &'de str) -> Result<T> {
+    pub fn get<'de, T: Deserialize<'de>>(&self, key: &str) -> Result<T> {
         // Parse the key into a path expression
         let expr: path::Expression = key.to_lowercase().parse()?;
 
@@ -161,7 +161,8 @@ impl Config {
         match value {
             Some(value) => {
                 // Deserialize the received value into the requested type
-                T::deserialize(ValueWithKey::new(value, key))
+                T::deserialize(value)
+                    .map_err(|e| e.extend_with_key(key))
             }
 
             None => Err(ConfigError::NotFound(key.into())),