feat: fix documentation, delete unused expression functions, replace errors on panics

sbroad-core/src/executor/ir.rs

sbroad-core/src/frontend/sql.rs

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

sbroad-core/src/ir.rs

sbroad-core/src/ir/api/parameter.rs

sbroad-core/src/ir/distribution.rs

sbroad-core/src/ir/explain.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/bool_in.rs

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

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

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

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