hsExprType :: HsExpr GhcTc -> Type

This is a less ambitious take on the same problem that !2182 (closed) and !3866 attempt to solve. Rather than have the hsExprType function attempt to efficiently compute the Type of every subexpression in an HsExpr, this simply computes the overall Type of a single HsExpr.

This is currently marked as a draft because there is at least one case that is known not to work: the case for HsTcBracketOut, which I have no idea how to implement. Other cases probably deserve further scrutiny as well.