aca37b65f1
Also stabilize constctlz for const ctlz_nonzero. The public methods stabilized const by this commit are: * `{i*,u*}::checked_pow` * `{i*,u*}::saturating_pow` * `{i*,u*}::wrapping_pow` * `{i*,u*}::overflowing_pow` * `{i*,u*}::pow` * `u*::next_power_of_two` * `u*::checked_next_power_of_two` * `u*::wrapping_next_power_of_two` (the method itself is still unstable)