libcore: Rename PI_2 to TWO_PI for consistency.

Rename `f32::consts::PI_2` and `f64::consts::PI_2` to `TWO_PI` for consistency with `Float::two_pi`. This deprecates `PI_2` as suggested by @aturon. There are still functions in `Float` that have digits in their name, but I think it is hard to avoid that.