File system API updates

• Start using rubyspec! See results
• Check for MRB_INT64 when building (currently doesn't understand conf.cc.defines, use conf.cc.flags instead)
• Add the Pathname module
• Add File::{basename,dirname,expand_path,absolute_path}
• Add Find module. (Raises an exception for Find.prune, since MRuby has no throw/catch)
• Dir.chdir defaults to $HOME
• Add Dir.open, Dir.new, Dir.read, Dir.close
• Other minor bugfixes to address some rubyspec results.