Unreturned futures should be executed

[neutralino1]

When I do this

@sematic.func
def foo() -> T:
    ...

@sematic.func
def bar() -> U:
    foo()
    ...

I expect foo to be resolved as part of graph. Currently it is not resolved because it is neither returned by bar, nor passed as an input to another Sematic Function.

We can solve this by having a global future registry that futures add themselves to upon instantiation.