Pathname#extname

[mble]

I'd like to take on Pathname#extname, as outlined in https://github.com/danielpclark/faster_path/issues/30

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• [ ] Write Rust implementation of Pathname#extname to FasterPath.extname
• [ ] Write refinements and monkey-patches for Pathname#extname and map implementation to FasterPath.extname
• [ ] Write tests from existing sources proving FasterPath.extname operates the same as Pathname#extname
• [ ] Write benchmarks in test/benches and compare the performance.
• [ ] Also compare both the original to the new method to verify compatibility.

[mble]

PR from @gogainda https://github.com/danielpclark/faster_path/pull/48 solves this.