Make OOM a lang item

[Gankra]

Currently alloc::oom just calls intrinsics::abort making the program look like it faulted, when really we killed it in a controlled manner. The reason it does that is that liballoc only assumes we have an allocator and no other OS/runtime.

However if oom was a lang item, we could implement it only in std, and have it print a message without allocating using

std::rt::abort("Allocator returned out of memory, aborting program")

no_std users could also opt into custom oom hooks, though they still have to crash the program.

We could also potentially add a flag for building std that makes oom unwind instead.

[alexcrichton]

Sounds like a good plan to me! This would basically use the same infrastructure as panic_fmt and friends.

[steveklabnik]

Isn't this more of an RFC kinda thing?

[steveklabnik]

Yes, seems like it would be. Let's go that route, please!