Implement Pathname#join

[danielpclark]

Rust seems to have it's own path joining methods. http://rustbyexample.com/std_misc/path.html This "may" do exactly what we want.

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

[mrageh]

@danielpclark I'd like to attempt adding FasterPath.join

[danielpclark]

Have at it :-)