A type is splittable if it has a splitting constructor that allows an instance to be split into
two pieces. The splitting constructor takes as arguments a reference to the original object,
and a dummy argument of type
split, which is defined by the library. The dummy argument
distinguishes the splitting constructor from a copy constructor. After the constructor runs,
x and the newly constructed object should represent the two pieces of the original
x. The library uses splitting constructors in two contexts:
Partitioning a range into two subranges that can be processed concurrently.
Forking a body (function object) into two bodies that can run concurrently.
A type X satisfies the Splittable if it meets the following requirements:
Splittable Requirements: Pseudo-Signature, Semantics