verify all basic types are concrete (and consider using sumtypes where possible)

[Krastanov]

Julia is not fast when working with containers over abstract types. In particular, the heap and other fields in the Simulation type are currently abstractly typed or have abstract eltypes. We need a thorough cleanup here, to ensure we do not introduce unnecessary slowdowns.

The first step is to verify that Simulation and its fields are all fully concrete (recursively into the type of each field). The biggest offender is the heap.

The second step would be to verify this for the rest of the major types introduced in the library.