Remove std::either

[alexcrichton]

Had to change some stuff in typeck to bootstrap (getting methods in fmt off of Either), but other than that not so painful.

Closes #9157

[Kimundi]

@alexcrichton I get the removal of Either, but why also Void? *Void and *() are not equivalent, the latter you can just construct, while the former is only possible to get with a raw pointer cast.

[glaebhoerl]

@Kimundi they are not equivalent and *() is correct. Void is the type that can't be constructed, and if an instance of it ever exists, it means the type system is unsound (if using safe code) or has been subverted (if using unsafe code). If you have a pointer-to-Void it implies that there's an instance of Void that it's pointing to, and you can call .uninhabited() on it and cause whatever havoc. The raw pointer half of it means that it requires unsafe code either way, so it's not as bad as it could be, but anyway, it's not the right idea.

(void in C-like languages is not the type-theoretical Void, it's actually equivalent to (). To avoid this confusion I was wanting to rename our Void to something like Never, but removing it is fine too.)

[alexcrichton]

From my understanding the point of Void is to allow these two to be equivalent:

fn foo() -> !;
fn foo() -> Void;

As @glaebhoerl pointed out, the Void type cannot be constructed, so neither of these functions can return. That being said, the compiler does no such analysis to realize that foo() cannot return in the second case, and that's supposed to be appeased with foo.uninhabited() (I think?).

We want to remove the std::util module, and I don't think that Void really has a place in the standard library (but I could be wrong)

[glaebhoerl]

FWIW I would rather just have ! be a first-class type. I recall this was discussed somewhere already, but not whether there were any obstacles mentioned.

[huonw]

Needs a rebase.

[brson]

I also don't quite understand the reason for removing Void. The original reason for adding it was to have an equivalent to C void pointers that wasn't c_void. Sometimes you just have an untyped pointer. It's true that we're not using it much but the few places where we were using it are incorrect to convert to () because derefing an arbitrary pointer to () is bogus. So I'm not convinced.

[brson]

I'm not sure how unbogus dereferencing a *Void is.

[alexcrichton]

Rebased to only remove std::either