A simple, fast, least-recently-used (LRU) cache implementation used for Servo's style system.
LRUCache uses a fixed-capacity array for storage. It provides O(1)
insertion, and O(n) lookup. It does not require an allocator and can be
used in no_std crates. It is implemented in 100% safe Rust.
By default, this crate won't need the standard library. However, if the std cargo feature is enabled, insert() will
return a replaced value which allows to reuse memory it may have contained.