doc: improve module documentation

src/executor.rs

src/executor/engine.rs

src/executor/engine/cartridge.rs

src/executor/engine/cartridge/cache.rs

src/executor/engine/cartridge/hash.rs

src/frontend.rs

src/frontend/sql.rs

src/frontend/sql/ast.rs

src/frontend/sql/tree.rs

src/ir.rs

src/ir/distribution.rs

src/ir/expression.rs

src/ir/helpers.rs

src/ir/operator.rs

src/ir/transformation/bool_in.rs

src/ir/transformation/equality_propagation.rs

src/ir/transformation/merge_tuples.rs

src/ir/transformation/redistribution.rs

src/ir/tree.rs

src/lib.rs