Complete and test the generalized_instances branch

[vittorioromeo]

The generalized_instances branch introduces a hierarchy of "system instances", allowing the users to create systems that directly act on component buffers (without going through "entity"). This can be useful for SIMD-like behavior - imagine storing c::position and c::velocity vectors in two huge separate buffers and simply adding them from beginning to end using SIMD operations.

The branch should also allow users to write systems that do not use components nor entities: this could be useful for "control" systems that need to be part of the DAG but that do not need to read/write component data.