Scheduler: Homogenization of statement_node members. Now only three members: {lhs, op, rhs}

This simplifies interfaces quite a bit and provides better encapsulation.
Mapping old members of statement_node to new members:
 lhs_type        -> lhs.type
 lhs_type_family -> lhs.type_family
 lhs.{union}     -> lhs.{union}    (no change)
 op_type         -> op.type
 op_family       -> op.type_family
 rhs_type        -> rhs.type
 rhs_type_family -> rhs.type_family
 rhs.{union}     -> rhs.{union}    (no change)

type and type_family could also be unified, but the current mechanism is considered sufficient.
Eventual changes at some later state could be reasonably well dealt with using string substitution.