Clean up the "utils" directory

[samth]

Some of these things belong in their own collection (contract combinators). Some belong sensible file names (cond-contracts). Some should be deleted from the code base (the require transformers).

[bennn]

I made a package for providing 2 values for 1 identifier, based on how TR protects typed identifiers in untyped code:
https://pkgs.racket-lang.org/package/choose-out

TR shouldn't use this package as-is (undo-choose-out-renaming would be a soundness hole). I just wanted to advertise it here.