opt(sbroad): fold constants under casts

sbroad/sbroad-core/src/executor.rs

sbroad/sbroad-core/src/executor/tests/concat.rs

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

sbroad/sbroad-core/src/ir/explain/tests/cast_constants.rs

sbroad/sbroad-core/src/ir/explain/tests/concat.rs

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

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