perf: speed up shard col positions lookup

sbroad-core/src/backend/sql/tree/tests.rs

sbroad-core/src/frontend/sql/ir/tests.rs

sbroad-core/src/ir.rs

sbroad-core/src/ir/expression.rs

sbroad-core/src/ir/operator.rs

sbroad-core/src/ir/operator/tests.rs

sbroad-core/src/ir/transformation/redistribution.rs

sbroad-core/src/ir/transformation/redistribution/groupby.rs

sbroad-core/src/ir/transformation/redistribution/tests.rs

...ore/src/ir/transformation/redistribution/tests/segment.rs