Add constants for f16 and f128 - Commit 1 adds associated constants for `f16`, excluding NaN and infinities as these are implemented using arithmetic for `f32` and `f64`. - Commit 2 adds associated constants for `f128`, excluding NaN and infinities. - Commit 3 adds constants in `std::f16::consts`. - Commit 4 adds constants in `std::f128::consts`.