Shared sets and sets with shared objects

[gdalle]

To avoid excessive allocations in Hessian tracing, we can make a new set type holding a reference to a shared set, such that every intermediate scalar quantity shares the same pattern (as opposed to having individual patterns).

This would require:

• Overloading trace_input so that each index x[i] points to the same shared set
• Handling the empty set intelligently to avoid re-creating shared sets in operations like similar and rand
• Once #103 is merged, overloading $\times$

Note that such shared objects would also be useful to implement tape-based recursive sets

[gdalle]

From the testing perspective, we would need to add functions with dead ends to highlight the differences:

• a shared Hessian pattern will include dead ends (see Walther 2008)
• individual Hessian patterns will ignore them

[adrhill]

Added in #135.