core: add as_str() for Level

[Folyd]

Motivation

Get the string representation of the Level is quite a common usecase.
Without this method, I normally need to implement it by myself, which
is fairly noisy.

#[inline]
fn level_to_str(level: &tracing::Level) -> &'static str {
    match *level {
        tracing::Level::TRACE => "TRACE",
        tracing::Level::DEBUG => "DEBUG",
        tracing::Level::INFO => "INFO",
        tracing::Level::WARN => "WARN",
        tracing::Level::ERROR => "ERROR"
    }
}

Solution

Add an as_str() method for Level. Similar to [log::Level::as_str()][1].

[1] https://docs.rs/log/0.4.14/log/enum.Level.html#method.as_str