Allow creation of Level and LevelFilter from usize

[WaDelma]

Currently Level::from_usize and LevelFilter::from_usize are private and cannot be used outside the crate.

I wanted to allow providing number via command line flag and use that to construct LevelFilter, but to do that I would need to replicate the code of those methods.

[KodrAus]

Hi @WaDelma 👋

I don't think we want to commit to an integer representation for these and the best approach would be for you to pick one for your use case and map them to log's. In my experience, integer representations, even well-understood ones like syslog, are an issue for interoperability because you've got no wiggle room for interpretation. You're just as likely to see 1 interpreted as error or as information.

[phil-opp]

I don't think we want to commit to an integer representation for these

Given that the types are repr(usize), the crate already commited to a specific integer representation in its public API. For example, the following prints 3:

fn main() {
    dbg!(log::LevelFilter::Info as usize);
}

(playground)

So I think it makes sense to expose the associated from_usize methods.