feature `relaxed_control_flow` is not additive

[RayMarch]

The relaxed_control_flow cargo feature is not additive in the way described in https://doc.rust-lang.org/cargo/reference/features.html#feature-unification (at least when FlowFn is part of the public API, however even if it is private, enabling this feature for all users having a shame dependency is undesirable, as it would remove control flow checks for everyone)

One possible solution would be to offer 2 modules, one for safe control flow and another for "unchecked" control flow, which requires the user to be aware of the graph-recording pattern and its implication on assignments (= operator).