Existing AST assumes the single-column expression is a special case of
multi-column expressions, so it cannot distinguish `c=(0)` from
`(c)=(0)`. This leads to incorrect behaviour and dtest failures. Fix
it by separating the two cases explicitly in the AST representation.
Modify AST-creation code to create different AST for single- and
multi-column expressions.
Modify AST-consuming code to handle column_name separately from
vector<column_name>. Drop code relying on cardinality testing to
distinguisn single-column cases.
Add a new unit test for `c=(0)`.
Fixes#6825.
Signed-off-by: Dejan Mircevski <dejan@scylladb.com>
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