Add support for enum as keys in maps

[mredaelli]

I found myself needing to deserialize something like:

enum Status {
   Doing, Completed
}
struct Format {
   // ...
}
struct Config {
   formats: HashMap<Status, Format>
}

and it didn't work out of the box.

I came up with this (just a few lines, plus some changes clippy insisted on).

I have zero experience with serde, so by all means let me know what you think.

[mockersf]

Thank you, and sorry it took me so long to get around to it. Looks good to me!

[mredaelli]

Thank you for the library! I wish HOCON were more used outside of Scala.