Support for literal type annotations

[kiranandcode]

I am not familiar enough with the internals to determine if this change is well-founded or fits within the design scope of effectful but opening a PR to discuss. This PR makes a small change to _simple_type to make Literal annotations at least work with uninterpreted ops and evaluate

from effectful.ops.syntax import ObjectInterpretation, implements
from effectful.ops.syntax import defop, NotHandled
from typing import Literla

@defop
def Price() -> Literal[5,10,20,50,80,120,150,200]:
    raise NotHandled

t = Price()
print(t)

with handler({Price: lambda: 10}):
    print(evaluate(t))

[eb8680]

Can you add some unit tests? You might also want to add Literal patterns for internals.unification.canonicalize and internals.unification.nested_type that do more or less the same thing.

[kiranandcode]

@eb8680 added one more toplevel test for unify working with literals, now done! ready to merge.