Access OpenType subscript and superscript glyphs

This small package replaces \textsuperscript and \textsubscript commands by equivalent commands that use OpenType font features to access appropriate glyphs if possible. The package also patches LaTeX's default footnote command to use this new \textsuperscript for footnote symbols. The package requires fontspec running on either XeLaTeX or LuaLaTeX. The package holds functions that were once parts of the xltxtra package, which now loads realscripts by default.