feat!: Use VerbosityFilter instead of log::Level

[joshka]

This makes it possible to later support the tracing crate directly
without needing to convert between log::Level and tracing::Level.

BREAKING CHANGE: The LogLevel trait now uses VerbosityFilter instead
of Option<log::Level>. This change should be source-compatible with
most existing application code using this crate, but breaks any code
that implements the LogLevel trait.

[coveralls]

Pull Request Test Coverage Report for Build 11875028263

Details

• 38 of 72 (52.78%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-10.3%) to 48.913%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib.rs	38	72	52.78%

Files with Coverage Reduction	New Missed Lines	%
src/lib.rs	3	48.91%

Totals
Change from base Build 11866209379:	-10.3%
Covered Lines:	45
Relevant Lines:	92

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💛 - Coveralls