Add comparison to fast-xml2js

[Rush]

https://github.com/cmrigney/fast-xml2js

[priezz]

I have not tried fast-xml2js, but xml2json (https://github.com/Cheedoong/xml2json) is much faster than xml-rs/treexml-rs pair used within this very module. Will put the numbers here later.

[priezz]

Benchmarked both modules, 5 passes for each. The time measured does not include the file load phase. File is about 138Mb and contains 1 736 790 lines.

Best pass for parser-xml2js - 21.76s
Best pass for xml2json - 5.21s