True if the value is a Just instance.
The .isJust field is deprecated in favour of the new Just.hasInstance(value) method.
The .hasInstance() version allow safely testing any value, even non-objects, and also
do union instance checking, rather than a simple tag check;
True if the value is a Just instance.
get [`is${name}`]() {
warnDeprecation(`.is${name} is deprecated. Use ${name}.hasInstance(value)
instead to check if a value belongs to the ADT variant.`);
return true;
}