Explore adding complex numbers as a type

[mplegendre]

We should explore adding complex numbers as an adiak type. They could be a formal type, or implemented as a tuple.

[baallan]

complex works very nicely as a formal type in c++ and is guaranteed therein to map to a C memory-adjacent pair of the same base type. so complexx[N] is in C double x[2N] and handling all by array pointer is happy.