Optimize checked_next_power_of_two consumers

[SomeFlyingThing]

Hint that checked_next_power_of_two results are powers of two or are at least as large as the input

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[khyperia]

r? libs

[clarfonthey]

Hmm, I can see these being useful, but I'm a little sceptical that the compiler is going to properly optimise this, given the complexity of the operations. (>= self is probably fine, though.)

Would you be willing to find some cases to test the codegen that are maybe a little less contrived? Like, perhaps trying to convert to an Alignment type without needing new_unchecked? That one also has an upper bound that similarly can't be reached by this method, so, it would be especially interesting to see if that works.