This MR is pure refactoring (#23916):
* Combine `HsLam` and `HsLamCase`
* Combine `HsCmdLam` and `HsCmdLamCase`

This just arranges to treat uniformly
   \x -> e
   \case pi -> ei
   \cases pis -> ie

In the exising code base the first is treated differently
to the latter two.

No change in behaviour.

More specifics:

* Combine `HsLam` and `HsLamCase` (constructors of `Language.Haskell.Syntax.Expr.HsExpr`) into one data construtor covering
  * Lambda
  * `\case`
  * `\cases`

* The new `HsLam` has an argument of type `HsLamVariant` to distinguish the three cases.

* Similarly, combine `HsCmdLam` and `HsCmdLamCase` (constructors of `Language.Haskell.Syntax.Expr.HsCmd` ) into one.

* Similarly, combine `mkHsLamPV` and `mkHsLamCasePV` (methods of class `DisambECP`) into one. (Thank you Alan Zimmerman.)

* Similarly, combine `LambdaExpr` and `LamCaseAlt` (constructors of `Language.Haskell.Syntax.Expr.HsMatchContext`) into one: `LamAlt` with a `HsLamVariant` argument.

* Similarly, combine `KappaExpr` and `ArrowLamCaseAlt` (constructors of `Language.Haskell.Syntax.Expr.HsArrowMatchContext`) into one: `ArrowLamAlt` with a `HsLamVariant` argument.

* Similarly, combine `PsErrLambdaInPat` and `PsErrLambdaCaseInPat` (constructors of `GHC.Parser.Errors.Ppr.PsError`) into one.

* Similarly, combine `PsErrLambdaInPat` and `PsErrLambdaCaseInPat` (constructors of `GHC.Parser.Errors.Ppr.PsError`) into one.

* In the same `PsError` data type, combine `PsErrLambdaCmdInFunAppCmd` and `PsErrLambdaCaseCmdInFunAppCmd` into one.

* In the same `PsError` data tpye, combine `PsErrLambdaInFunAppExpr` and `PsErrLambdaCaseInFunAppExpr` into one.

p* Smilarly combine `ExpectedFunTyLam` and `ExpectedFunTyLamCase` (constructors of `GHC.Tc.Types.Origin.ExpectedFunTyOrigin`) into one.

Phew!