Implement conversions between Option<Level> and LevelFilter

[joshka]

This is nice to have when working with the log crate, as it allows
for more ergonomic conversions between the two types. Specifically, This
allows type bounds that require Into or Into<Option>
to be used. This in turn makes it possible to write more generic code
that can work with tracing and log crates interchangeably.

Specifically, this supports some ideas in
clap-rs/clap-verbosity-flag#121

[tisonkun]

While From<LevelFilter> for Option<Level> is trivial and straightforward, None to LevelFilter::Off may not. I suppose different system may have different opinion for the "default" level filter.

[tisonkun]

You may write:

opt_level_filter.unwrap_or(LevelFilter::Off).into()

[Thomasdezeeuw]

I'm sure this is always going to be obvious.

[joshka]

Did you leave out "not" or are you disagreeing with the comment above here and saying that this is actually fine and a reasonable choice to convert Option<Level>::None to LevelFilter::Off?

[joshka]

While From<LevelFilter> for Option<Level> is trivial and straightforward, None to LevelFilter::Off may not. I suppose different system may have different opinion for the "default" level filter.

Is None indicating a default, or the absence of a level? The benefit of making None -> Off is that it round trips. Perhaps a doc comment about this would be sufficient.

You may write:

opt_level_filter.unwrap_or(LevelFilter::Off).into()

There's no Option<LevelFilter> in this change. Can you clarify what you meant here?

[joshka]

BTW, I solved the underlying need for this through another means (changed the trait associated type bounds), so I'm unblocked from an implementation perspective for now at least.