Implement Pathname#children

[danielpclark]

• [x] Write Rust implementation of Pathname#children to FasterPath.children
• [x] Write refinements and monkey-patches for Pathname#children and map implementation to Pathname.new(result)
• [x] Write tests proving FasterPath.children operates the same as Pathname#children
• [x] Write benchmarks in test/benches and compare the performance.
• [x] Copy benchmarks to test/pbench/pbench_suite.rb