Remove Hash#key_index

[asterite]

As I mentioned in #9952 (comment) I needed this for the compiler and added it to Hash, a really long time ago. I don't think that method is generally useful, and it was only used in one place in the compiler. Ruby doesn't have such method, so let's remove it.

As a side note, I'd also like to remove Hash#first_key and Hash#first_value: they are redundant because one can do hash.first[0] and hash.first[1] (plus they don't exist in Ruby either).

[j8r]

If first_key and first_value are removed, last_key and last_value have to be too, and all their nillable ? variants.

[asterite]

Yes. They make the API pretty ugly, in my opinion, for a very little gain.

[straight-shoota]

Github search showed this method is probably not much in use. Although I use it in crinja, but that can be changed to the same workaround as in the compiler. It should be deprecated instead of direct removal.

Still I'm not 100% convinced that it should really be removed. It is useful not only for the compiler.

[asterite]

Yeah... actually, it is useful, and the other methods too. Now that I think about it, we can use index from enumerable for this, right?

[jkthorne]

Should this have a depreciation warning? I am just concerned about upgrading versions and having broken code. I dont know if that is as big of a concern before 1.0.

[asterite]

1.0 will be the last version that will introduce breaking changes. It already includes a bunch of them, I don't think removing this is a big deal. The code will just fail to compile and it's very easy to make it work again.