Make some float methods unstable const fn

[eduardosm]

Some float methods are now const fn under the const_float_methods feature gate (except f16 and f128 versions, which I placed under f16 and f128 feature gates respectively).

In order to support min / max, the implementation of some intrinsics had to be moved from Miri to rustc_const_eval (cc @RalfJung).

If this is accepted, a tracking issue should be created (for now I set issue = "none").

impl <float> {
    // #[feature(const_float_methods)]
    pub const fn recip(self) -> Self;
    pub const fn to_degrees(self) -> Self;
    pub const fn to_radians(self) -> Self;
    pub const fn max(self, other: Self) -> Self;
    pub const fn min(self, other: Self) -> Self;

    // #[feature(float_minimum_maximum)]
    pub const fn maximum(self, other: Self) -> Self;
    pub const fn minimum(self, other: Self) -> Self;

    // Only f16/f128
    pub const fn is_sign_positive(self) -> bool;
    pub const fn is_sign_negative(self) -> bool;
    pub const fn next_up(self) -> Self;
    pub const fn next_down(self) -> Self;
}

r? libs-api

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri