Add `no_std` support to `bevy_log`

[Shatur]

What problem does this solve or what need does it fill?

Many crates use logging and in order to support no_std, it's required to manually depend on log because tracing doesn't work on no_std yet.

What solution would you like?

It would be convenient to cleave the bevy_log crate with a tracing feature.

[hukasu]

this makes it seem that it does work on no_std https://docs.rs/tracing/latest/tracing/#crate-feature-flags , unless depending on liballoc is a blocker