The `SplaySetFn` functor

The `SplaySetFn` functor provides a splay-tree implementation of the `ORD_SET` signature parameterized over the element type.

It is recommended, however, that one use the `RedBlackSetFn` functor instead of `SplaySetFn`, since experimentation has shown it to be faster across the board.

## Synopsis

``functor SplaySetFn (K : ORD_KEY) :> ORD_SET where type Key.ord_key = K.ord_key``