This adds two new types to the `de::value` module,
`BorrowedStrDeserializer` and `BorrowedBytesDeserializer`. A
`BorrowedStrDeserializer` is just like `StrDeserializer`, except the
lifetime of the string is tied to the lifetime of the deserializer. This
can be useful when, for example, deserializing into a
`HashMap<&str, &str>` when the keys/values are tied to the deserializer
itself.
The `BorrowedBytesDeserializer` has no analog, but it's the same as
`BorrowedStrDeserialize` except for `&[u8]` instead of `&str`.
