Simplify `CASE`

[ranma42]

The expressions are equivalent (the arrow describes the preferred direction):

CASE
  WHEN t1 THEN r1
  ELSE CASE
    WHEN t2 THEN r2
    ELSE r3
  END
END

→

CASE
  WHEN t1 THEN r1
  WHEN t2 THEN r2
  ELSE r3
END

To keep the following expressions simpler, they are given using ternary notation, i.e. t1 ? r1 : r2 instead of CASE WHEN t1 THEN r1 ELSE r2 END.

[ ] t ? x : x → x
[x] (t ? true : false/null) ? r1 : r2) → t ? r1 : r2 dotnet#34175
[x] (t ? false/null : true) ? r1 : r2) → !t ? r1 : r2 dotnet#34175
[ ] t ? true : false → t == true
[ ] t ? false : true → !boolExpr
[ ] !boolExpr ? x : y → boolExpr ? y : x
[ ] !(t ? x : y) → t ? !x : !y

WIP