Explicitly implement Borrow for String & Vec for non-nightly
Explicitly implement Borrow for String & Vec for non-nightly

While it isn't as fully featured as the nightly version, this allows
using borrowed keys for `get` for what I believe are the most common
cases: `Borrow<str>` and `Borrow<[T]>`. I believe this is also backward
compatible with eventually enabling `Borrow<D> where K: Borrow<D>`.
jeromefroe/lru-rs