Revert an unnecessary change to SWIG bindings.
Fixes for cola::FixedRelativeConstraint.

This commit includes 3 changes:
 -  Alters FixedRelativeConstraint to take a vector of rectangle indices
    rather than a set, so that this constraint may be used from SWIG bindings.
 -  Fixes a bug in FixedRelativeConstraint::generateSeparationConstraints()
    where the wrong memory address for variables was sometimes being set
    for separation constraints.
 -  Fixes a bug where IDs were not properly getting remapped for all compound
    constraint types when CompoundConstraint::updateVarIDsWithMapping() was
    called.