Remove ValueWithKey struct

[gnieto]

While using this library, I end up having lifetime issues with
Config::get. I've seen that current implementation forces the caller
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 not 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 purpose 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

[mehcode]

🙇‍♂️ Thank you for your contribution. I apologize for the late response. Moving forward I want to re-think the design of config as a whole. See #111 for more details.

---

If you rebase it I'll merge it and put it out in a patch version. That type definitely needs to go.

[gnieto]

@mehcode rebased!

I'll try to add some feedback during next week on #111 regarding how I plan to use this library, if it's useful. Once you are comfortable with a design, I'm willing to keep contributing.

For my use case, it has been very easy to integrate this library. I've written a custom Source and set up all the stack with very low effort. Thanks for you work!