Make equality constraints in kinds invisible

Issues #12102 (closed) and #15872 (closed) 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 (#12102 (closed) (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 AnonArgFlags 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 #12102 (closed) (comment 136220).

Fixes #12102 (closed) and #15872 (closed).