Implement CASE expression.

src/backend/executor/execQual.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/outfuncs.c

src/backend/optimizer/plan/createplan.c

src/backend/optimizer/util/clauses.c

src/backend/optimizer/util/tlist.c

src/backend/optimizer/util/var.c

src/backend/parser/analyze.c

src/backend/parser/gram.y

src/backend/parser/keywords.c

src/backend/parser/parse_coerce.c

src/backend/parser/parse_expr.c

src/backend/parser/parse_target.c

src/backend/rewrite/rewriteHandler.c

src/include/nodes/nodes.h

src/include/nodes/parsenodes.h

src/include/optimizer/clauses.h