Add merge builder for raise

[kyay10]

I re-implemented some of Effect/s methods using the merge builder to check its functionality.

Note for the future: when/if union types are in the language, merge should likely be deprecated in favour of a union builder that has separate types for the returned and raised types (so that they're inferred separately) and returning a union of both.

[serras]

I don't fully understand why this works...

[kyay10]

TL;DR: nullable is just secretly a restricted merge because it's taking Raise<Nothing?>.() -> A and returning A?, which is the supertype of Nothing? and A.
Note also that instead of { null } in the original code, you could've had ::identity, and hence merge(blah) == fold(blah, ::identity, ::identity)

In other words, nullable is a general example of returning the common supertype of the raised and the returned (except of course that we have NullableRaise to provide nice binds for related types like Option due to the lack of context receiver support right now)

Side note: In the future when we have union types, this pattern would work with a lot more types, but right now, this just utilises the fact that ? is the only natively supported union type in Kotlin. Btw, I think the design for union types prevents them from being automatically inferred (i.e. the compiler won't infer a type parameter T to a union type ever, but it respects explicit mentions of union types) and so a union builder that infers the types separately and returns a union of them would be the way to go when that happens

[serras]

I am amazed by the fact that you could find this (cool) approximation 🤯

[kyay10]

With a powerful type system, the possibilities are infinite! 🚀

[franciscodr]

Thanks, @kyay10! Amazing contribution 🚀