Provide source file in metadata to transforms

While I’ve generally resisted doing this, since tranforms _should_
generally always be looking at the parsed form of the file and not the
original, having access to this property is necessary in some cases to
enable embedding the original source file within the tranformation
result (e·g, when the source file is X·M·L, and there are parsers which
preprocess it). When the source file is _not_ X·M·L, this property is
probably use·less, but included never·the·less.