Implement slog::Value for std::sync::LazyLock

[TheJokr]

This impl allows using LazyLock like an FnValue. The notable difference is that LazyLock memoizes the function's result, whereas FnValue generates a fresh result every time. This is relevant when values are stored in a Logger for multiple log calls.

Closes #360

[Techcable]

Thank you! I will squash & merge this before the v2.9.0 release :)