Change overflowing_(add|sub|mul) to be non generic

[strega-nil]

And also rename them to T_wrapping_(add|sub|mul), in order to allay confusion
with (add|sub|mul)_with_overflow.

[rust-highfive]

r? @alexcrichton

(rust_highfive has picked a reviewer for you, use r? to override)

[hanna-kruppe]

Wow, this is a pretty impressive diffstat. So large, in fact, that GitHub refuses to display a diff for libcore/num/mod.rs 😠 Could you please explain the motivation for this change and briefly summarize what all the diff noise in libcore/num/mod.rs is about?

[nagisa]

Ditto #29288 (comment)

[killercup]

You can see the full diff here, by the way: https://github.com/rust-lang/rust/pull/29292.patch

[strega-nil]

@rkruppe It's not actually an impressive diff. It's a six line change, except that I need to stage0 that change inside a 500 line macro...

[strega-nil]

If you have a better idea of how to implement it, I am all ears.

[strega-nil]

I would also be happy to implement it like simd_add. I honestly didn't know that was an option.