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Issues #12102 and #15872 revealed something strange about the way GHC handles equality constraints in kinds: it treats them as _visible_ arguments! This causes a litany of strange effects, from strange error messages (ghc/ghc#12102 (comment 169035)) to bizarre `Eq#`-related things leaking through to GHCi output, even without any special flags enabled. This patch is an attempt to contain some of this strangeness. In particular: * In `TcHsType.etaExpandAlgTyCon`, we propagate through the `AnonArgFlag`s of any `Anon` binders. Previously, we were always hard-coding them to `VisArg`, which meant that invisible binders (like those whose kinds were equality constraint) would mistakenly get flagged as visible. * In `ToIface.toIfaceAppArgsX`, we previously assumed that the argument to a `FunTy` always corresponding to a `Required` argument. We now dispatch on the `FunTy`'s `AnonArgFlag` and map `VisArg` to `Required` and `InvisArg` to `Inferred`. As a consequence, the iface pretty-printer correctly recognizes that equality coercions are inferred arguments, and as a result, only displays them in `-fprint-explicit-kinds` is enabled. * Speaking of iface pretty-printing, `Anon InvisArg` binders were previously being pretty-printed like `T (a :: b ~ c)`, as if they were required. This seemed inconsistent with other invisible arguments (that are printed like `T @{d}`), so I decided to switch this to `T @{a :: b ~ c}`. Along the way, I also cleaned up a minor inaccuracy in the users' guide section for constraints in kinds that was spotted in ghc/ghc#12102 (comment 136220). Fixes #12102 and #15872.
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