Add TAU consts to f32 and f64

[icefoxen]

Where tau = 2 * pi.

In aesthetic terms, very much in the vein of ha-ha-only-serious, the argument is described here.

In practical terms, we already have const's for pi divided by various useful integers, so why not include multiplying it by the only really useful integer as well?

PR is trivial and already implemented in icefoxen@50173bd , but apparently I need a feature name and issue number and such associated with it, hence this ticket.

[tesuji]

Python and C/C++ doesn't seem to include that constant either.

[memoryruins]

Python 3.6 added a tau constant, math.tau. Nim and Perl 6 include one as well.

[hellow554]

@icefoxen without any guarantees, I'm sure it's pretty safe to just open a PR with an explanation and some reasoning and see what happens. To open an issue for asking is not necessary, just a tracking issue afterwards will be needed.

[ollie27]

See also: rust-lang/rfcs#1486

[Dylan-DPC-zz]

Closing this as it is easier to create a tracking issue when the rfc is accepted. Thanks

[est31]

Tracking issue: #66770

[steveklabnik]

For posterity's sake, this was rejected back in #15248; any new RFCs would have to address that discussion.