Leverage static str key when possible

[tisonkun]

Replace &'k str in Key<'k> with MaybeStaticStr<'k> to leverage &'static str when in most cases it is.

Open for review, especially on:

1. Whether it is backward compatible.
2. Whether MaybeStaticStr causes significant indirection.

Otherwise, for downstream append, even if the key is a static str, one may have to key.to_string to resolve some lifetime issue. See:

• https://github.com/open-telemetry/opentelemetry-rust/blob/f7b0dd999d3d6434a6e52291efccc98e9dfcea81/opentelemetry-appender-log/src/lib.rs#L216 (they have Key::from_static_str)
• https://github.com/slog-rs/stdlog/blob/252b6bb6435a8a3ba44748b2e1626aa676508097/kv.rs#L46 (they have slog::Key::from(static_str))

[KodrAus]

This looks like a good change to me, thanks @tisonkun!

[KodrAus]

As noted, we should call the method from_str_static, to match the convention of other methods in log

[KodrAus]

Thanks @tisonkun