rename `std::num::Zero` and `std::num::One`

[liigo]

std::num::Zero means neither "the number 0"(noun) nor "zero a value"(verb).
std::num::One does not means "the number 1" (or once).
They are both ambiguous and confusing. There are many comments here:
https://mail.mozilla.org/pipermail/rust-dev/2014-April/009432.html

std::num::Zero should be renamed to std::num::AddIdentity,
std::num::One should be renamed to std::num::MulIdentity,
to match their original meanings.

(Or put them in a separate module: std::num::identity::{Zero, One}.)
(Or we remove them entirely from std.)

[liigo]

Is there any objection? If not, I'll submit a PR to rename them.
@alexcrichton @huonw

[alexcrichton]

I personally favor the names Zero and One over AddIdentity and MulIdentity.

[huonw]

I have no interest in this renaming.

In mathematics, the additive/multiplicative identity is very often called 0 and 1, even for things that aren't conventionally "numbers", e.g. vector spaces and groups.

[liigo]

@alexcrichton How about we move Zero and One to submodule std::num::identity? In this way, we still call it Zero and One, but also decrease ambiguous.

[alexcrichton]

What is the impetus for this renaming? Is it because the words zero and one are ambiguous?

[liigo]

Because it is Add/Mul Identity, not is number 0/1. Although it's very often "called" zero/one, but it not "is".

[steveklabnik]

This kind of change would go through the RFC process today. Please open one if you're interested in persuing this change!

[liigo]

For ones who comes here:
At the end, Zero and One was deprecated since 1.11, with a reason 'no longer used', in #34530. Jun, 2016.
It takes over a year to make decisions about them. CC #27739.

[matt2xu]

It takes over a year to make decisions about them.

I wanted to "Add my reaction" to your post, but I cannot say whether it should be 😕 or 😄 ^^