Rename or merge GHC.JS.{Core,STG,}Utils

One thing I often do to open GHC.Core.Utils is to type CoreUtils into my fuzzy file finder. This became a lot more tedious since we merged the JS backend, because now the first match is GHC.JS.CoreUtils. It is similar for GHC.Stg.Utils, but I don't search for it that often.

Hence I propose to merge those modules into a single GHC.JS.Utils module. They aren't that big to begin with (perhaps 600 lines after the merge).

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