suggestion: Make directives public in env_filter

[AdamGS]

My use case is using env_filter to configure another logger (pyo3_log). Right now I can only use the top-level filter but if I could access each directive in the Filter instance, it would be trivial to use it in every logger that doesn't have its own utility to parse standard directives.

If the maintainers think this is a good idea, I'm glad to do the actual work.

[epage]

Could you help me understand why you need access to the individual built directives, rather than using env_filter::Filter to do the filtering within your logger?