QualifiedStrings (#26503)

Implements the new -XQualifiedString extension. Adds new HsQualLit/QualLitPat constructors in HsExpr/HsPat, respectively, for qualified strings and other potential qualified literals. During the renamer phase, these constructors get expanded to M.fromString "string literal" and ((M.fromString "string literal" ==) -> True), so typechecking should proceed as normal.

While working on this MR, I had to make some updates to the underlying proposal: https://github.com/ghc-proposals/ghc-proposals/pull/742

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