stabilize const_result

[RalfJung]

Waiting for FCP to complete in #82814

Fixes #82814

[rustbot]

r? @tgross35

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[tgross35]

Cc @rust-lang/wg-const-eval for somebody else to double check the use of const_precise_live_drops, otherwise lgtm.

[tgross35]

Thanks @lcnr

This just changes const gates on the following:

impl<T, E> Result<T, E> {
    pub const fn as_mut(&mut self) -> Result<&mut T, &mut E>;
}

impl<T, E> Result<&T, E> {
    pub const fn copied(self) -> Result<T, E>
    where T: Copy;

impl<T, E> Result<&mut T, E> {
    pub const fn copied(self) -> Result<T, E>
    where T: Copy;

impl<T, E> Result<Option<T>, E> {
    pub const fn transpose(self) -> Option<Result<T, E>>;
}

Which is everything described at #82814.

r=me after FCP