Trait that marks a type as being representable as a PaxAny, and provides
the implementation for going to/from that type. For all builtins this
means going to/from a pax value. For others to a Box<dyn Any>. This
is automatically Implemented for PaxValue types through the macro
impl_to_from_pax_value!, and for other types by implementing the marker
trait ImplToFromPaxAny.
Checks the opt-in value-roundtrip contract for typed one-way bindings.
Recursive type graphs conservatively fall back to conversion. This tracks
only the current capability check, never values or property lifetimes.