effect -- abstract parent feature for effects
effect provides a means to perform effectful operations. Instances
of effect are installed in the current environment while their code is
executed. The code may access the effect via <type>.env.
replace effect in the current context by this and abort current execution
execute code provided in f.call while this effect is installed
in the current environment. Return immediately in case abort is
NYI: uses type parameter T only to simplify C backend
abort the current execution and return from the surrounding call to
abortable with result == false.
execute the code of 'f' in the context of this effect