Reduce number of congruence rules applied by `pre_dcp`

Currently, every rewrite step (except objective function pre-compositions) results in an application of a (custom) congruence rule. This is unnecessary. All the rewrites on a given component can be applied together.

Before: (`rewrite_constraint_2`; `rw_x`), (`rewrite_constraint_2`; `rw_y`), (`rewrite_constraint_2`; `rw_z`), ...

After: `rewrite_constraint_2`; (`rw_x`; `rw_y`; `rw_z`), ...