Logforth RUST_LOG Filter

This filter is derived by env_filter, with significant modifications to suit our needs:

1. Logforth needs not the original FilterLog struct.
2. Logforth would use its own Level, LevelFilter, FilterCriteria, and Record types.
3. The regex based global filter is discarded. Filtering by targets should be sufficient.
4. Interfaces and methods are refactored to be more ergonomic.