Add a fallabck mechanism for Dynamic Objects method invocation
The API is just wrapping a define call.
Previously, the invokers produced a MethodHandle representing the
target. Now, if the DynOb detects a fallback setted, the target are
guarded.
The guard test is just checking if the aked property exists. If it does
exits then the target will be called as usual, if is not, the fallback
will be triggered.