Adds M_TAU constant for τ (2π)

This makes dealing with radians much easier:

```php
    $1 = M_TAU; # full turn (360˚)
    $2 = M_TAU / 2; # half-circle (180˚)
    $4 = M_TAU / 4; # quarter-circle (90˚)
    $8 = M_TAU / 8; # eighth-circle (45˚)
```