[#665] Apply dipDrop2swapDrop in a separate stage, and refactor how stages work

Description

Problem: dipDrop2swapDrop rule blocks adjacentDIPs.

Solution: First run main stage rules without dipDrop2swapDrop, then run them plus dipDrop2swapDrop. dipDrop2swapDrop can open up opportunities for further optimization, hence it needs to be run with other rules in tandem.

Problem: Currently, optimizer rules are represented as list of stages, where each stage is constructed as orSimpleRule flattenSeqLHS .... This makes the architecture not very pluggable.

Solution: Represent the ruleset as a newtype over IntMap (NonEmpty Rule). Add utilities for working with this type. When running the rules, fold the NonEmpty representing the stage using orSimpleRule with flattenSeqLHS as initial.

Related issue(s)

Resolves #

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