Move spawn() from capacity DefaultContextInteractions to Lifecycle

[gallandarakhneorg]

The function spawn() declared in the capacity DefaultContextInteractions may be declared into the Lifecycle capacity.

[gallandarakhneorg]

It may be useful to use the variadic parameter and/or parameter default value in the definition of the built-in capacities.
For example:

def emit(e : Event, scope : Scope<Address> )
def emit(e : Event)

may becomes:

def emit(e : Event, scope : Scope<Address> = null )