math: add Tau in constants

[njourdane]

Add Tau in mathematical constants.

In many cases, formulas can be more elegant using the Tau constant instead
of Pi. You will find a good number of arguments and explanations
in the Tau manifesto, which describes Tau as:

the most important numbers in mathematics, perhaps the most important:
the circle constant relating the circumference of a circle to its linear
dimension. For millennia, the circle has been considered the most
perfect of shapes, and the circle constant captures the geometry of the
circle in a single number.

The constant is based on the corresponding page in the online
encyclopedia of integer sequences, and is also
equal to math.Pi * 2.

Usage:

perimeter := math.Tau * radius

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[Goutte]

Thanks so much for this !

I've been missing it since the beginning, but I was too lazy to do it myself.

As I stated before, TAU is the circle constant, and PI the disk constant.

When civilizations make mistakes, the inertia is heavy.

[ALTree]

The proposal was rejected (#40663) so I'm closing this PR.