Implement can Effect syntax for annotating effects

[jfecher]

The effects3 branch is currently under development for implementing effects in ante's backend. It is expected that the type system component of effects is already complete but a crucial part is missing - the ability to annotate the expected effects a function has.

A function should be able to specify its effects via a new can keyword:

foo (x: I32) : I32 can Effect1, Effect2 =
    effect1 x
    effect2 x

If a function's return type is inferred, the function's effects are also inferred:

foo (x: I32) =
    effect1 x
    effect2 x

If a function's return type is specified but no effects are specified, we should assume the function is meant to be pure rather than inferring the effects:

foo (x: I32) : I32 =
    // Error: effect Effect1 is unhandled
    effect1 x
    // Error: effect Effect2 is unhandled
    effect2 x

[jfecher]

Parsing for this issue has been implemented by #175. Next up is integration into name resolution and type checking for these annotations. E.g. name resolution should ensure the effects are declared names, and type inference can check this set against the inferred set to issue an error if the inferred set is not a subset of the declared set.

Open question: Should ante issue a warning during type inference if the inferred effect set is a subset of the declared set instead of equal to it? My current thinking is that this would discourage use of effect aliases like IO which contain several effects in them that may be unused.