Move `log2` to `Crypto.Number.Basic` and export it

I found the need for `log2` while implementing montgomery ladder multiplication in `crypto-pubkey-types`. To export it I considered it makes more sense to have it in `Basic` instead of `F2m`, but if you think it's better in other module or not have it exported at all, and redefine it in `crypto-pubkey-types`, I can still do so.