Add a promise object that allows Eolian interface to include promises
as a way to have asynchronous value return and composibility.
The usage is like this in a .eo file:
class Foo {
methods {
bar {
params {
promise: Promise<int>;
}
}
}
}
Which will create the following API interface:
void foo_bar(Ecore_Promise** promise);
and the equivalent declaration for implementation.
However, the API function will instantiate the Promise for the
user and the implementer of the class.