Add ensure_pow method

[lemunozm]

This PR extends #12967 with the ensure_pow function. I think this is also an interesting function to have along with the ensure_ops family methods.

ensure_pow acts similarly to checked_pow but returns an ArithmeticError instead.

Related #12754

[ggwpez]

Please add some minimal success tests or merge in #13043 and use the test structure from there.

[lemunozm]

I've implemented ensure_pow as a function because checked_pow is also implemented as a function. But maybe can be interesting to follow the same pattern as other ensure_ops and make a trait EnsurePow with an ensure_pow method. What do you think? I'm not sure why in num-traits the checked_pow is not implemented as a trait TBH.

[ggwpez]

I've implemented ensure_pow as a function because checked_pow is also implemented as a function. But maybe can be interesting to follow the same pattern as other ensure_ops and make a trait EnsurePow with an ensure_pow method. What do you think? I'm not sure why in num-traits the checked_pow is not implemented as a trait TBH.

Dont know what is better here. Maybe there can be types that are not CheckedMul but can still have an ensure_pow?
I cant imagine but that would not work with a free-standing function otherwise.

[lemunozm]

Added tests. I think all pending comments are "solved" for now.

I've opened an issue rust-num/num-traits#254 trying to get more info in order to know the best possible design decision for doing it as a function or as a trait method. But I think is not blocking, so if you are ok with it right now, it's ok for me.

[ggwpez]

(Looks like the CI does not notice when you dismiss reviews 🤔)

Merging now since there were no changes since Basti last reviewed.

[ggwpez]

bot merge

[lemunozm]

(Looks like the CI does not notice when you dismiss reviews 🤔)

Yes, it was weird, the review button switched between you and your partner, instead of selecting both 🤷‍♂️.

Anyways, thanks for your reviews! 🙌🏻

[Polkadot-Forum]

This pull request has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/polkadot-release-analysis-v0-9-37/1736/1