Note [Drop unfoldings and rules]

We want to drop the unfolding/rules on every Id:

  - We are now past interface-file generation, and in the
    codegen pipeline, so we really don't need full unfoldings/rules

  - The unfolding/rule may be keeping stuff alive that we'd like
    to discard.  See  Note [Dead code in CorePrep]

  - Getting rid of unnecessary unfoldings reduces heap usage

  - We are changing uniques, so if we didn't discard unfoldings/rules
    we'd have to substitute in them

HOWEVER, we want to preserve evaluated-ness;
see Note [Preserve evaluatedness] in GHC.Core.Tidy.