Log Message:
Update tex-fontspec{,-doc} to 2.1e.

Changes:
    * Internal changes for luatexbase v0.3.
    * Bug fix when \itdefault is "sl" rather than "it".
      E.g., when using the "slides" class.
    * Minor internal changes, including merging some code from unicode-math.
    * New documentation for defining custom kerning and ligatures
      when using LuaLaTeX.
    * Fix bug when defining bold italic fonts by filename.
    * Avoid infinite loop when the Latin script is requested for a font
      that does not contain it. TODO: a suitable fallback script should be
      chosen; right now we just ignore the script selection.
    * Fix for bug introduced in the last release:
      small caps weren't being automatically selected correctly
    * Fix for colours bug introduced in the last update
    * Fix for for bad interaction with LuaLaTeX and fallback fonts
      (such as using \slshape when no slanted font specified)
    * Behaviour/messaging improvement when scripts/languages are requested
      that do not exist in the font
    * Fix bug with detecting font features/scripts in some cases with:
        - `\fontspec_if_feature:n(TF)`
        - `\fontspec_if_language:n(TF)`
        - `\fontspec_if_current_script:n(TF)`
        - `\fontspec_if_current_language:n(TF)`
    * Some messages in the log file are improved
    * Code for "visible space" fixed for LuaLaTeX use
    * Lots of internal changes to bring the implementation closer
      to being "native expl3".