Marker trait. Implement only for types that are not part of PaxValue, but
need to be stored inside a PaxAny. If they are part of pax value, instead
implement CoercionRules manually, or using the default impl macro as seen
in coercion_impls.rs
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. 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.
This trait is implemented by all types that has a bultin equivalent
representation (see to_from_impls module) This is NOT responsible for
coercing between types, but returns an err in all cases where the underlying
type is not exactly what is expected