toNullable() specialization.

[artem-zinnatullin]

Recent PR #20 made me rethink toNullable(), I think it needs to be abstract with specialized implementation in Some an None.

That gives following benefits:

• No instanceof check, likely JIT or AOT were specializing the function for us, but now we do it at compile time
• Proper static nullabily in Some.toNullable(). It lets compiler statically figure nullability if type is known to be Some

Note that because of nullability change in Some.toNullable() the change is API incompatible, but ABI compatible so we're still allowed to release it in a minor update.

cc @Egorand, @AlecStrong

[AlecKazakova]

interesting...i wonder if this would mean component1 on None would have proper type Nothing? without having to make component1 abstract

[artem-zinnatullin]

Without abstract component1 None won't have component1 on its own, it's not a data class right

With abstract component1 however we're getting proper static nullability in both Some and None because it's declared as nullable in Optional so it works well for None and then compiler adds @NotNull to Some's override so it sees it statically as non-null. I mentioned that in https://github.com/gojuno/koptional/pull/20#discussion_r189455048 but maybe wasn't clear enough :)

[artem-zinnatullin]

As for component1(), I don't think it makes sense in None, so it should be specific to Some.

But #20 adds component1 to None (we can continue this discussion there)

[artem-zinnatullin]

@ming13 @dmitry-novikov @nostra13 can you PTAL ыы?