Optimise typeKind and TyCon

This ticket records two ideas

1. TyCon is a sum of six constructors. That means that even tyConUnique requires a six-way branch to get the unique. Moreover quite a few fields are common across all the constructors: Name, Unique, [TyConBinder] etc. So it might be better to represent TyCon as a tuple of all its common fields, with the last field being a 6-way sum for the details.

   See branch wip/T21623-tycon and !9351 (closed). See also #22503 (closed)

   Status: done, landed

2. typeKind is a heavily used function, but I think it can be made faster. Notably

   typeKind (TyConApp tc tys) = piResultTys (tyConKind tc) tys

   does more work than necessary, esp in the case where the TyCon is saturated and has a closed kind.

   Status: stalled:: I have a branch wip/T21623-faster but puzzlingly it doesn't go faster; it goes slower! I lack the cycles to pursue this, so I'm parking it for now