This branch prepares `tracing-log` for an 0.1 release.

Taking this crate out of alpha will allow us to publish
`tracing-subscriber` 0.1, as that crate depends on `tracing-log` & I
don't want to publish an 0.1 that depends on an alpha crate.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>