- allow multiple WHEN statements
- allow for syntax like `CASE x WHEN 1 THEN 2 WHEN 3 THEN 4 ELSE 5 END`
NOTE: This changes also the way the CASE operator is stored:
- CASE [expr] exprList [ELSE expr2] END
- exprList holds each of the WHEN statements with:
expr := WHEN, expr2 := THEN
Added also tests in test/select_tests.cpp
and adapted the existing one to reflect the new storage
daf8fe7a45