Move {f32,f64}::clamp to core.
`clamp` was recently stabilized (tracking issue: https://github.com/rust-lang/rust/issues/44095). But although `Ord::clamp` was added in `core` (because `Ord` is in `core`), the versions for the `f32` and `f64` primitives were added in `std` (together with `floor`, `sin`, etc.), not in `core` (together with `min`, `max`, `from_bits`, etc.).
This change moves them to `core`, such that `clamp` on floats is available in `no_std` programs as well.
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