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, 1 insertions, 1 deletions

diff --git a/src/ser.rs b/src/ser.rs
index 32b99ac..b4c1628 100644
--- a/src/ser.rs
+++ b/src/ser.rs
@@ -3,7 +3,7 @@ use std::fmt::Display;
 use std::mem;
 
 use error::*;
-use value::{Value, ValueKind, ValueWithKey};
+use value::{Value, ValueKind};
 use Config;
 
 #[derive(Default, Debug)]