@subentry now implemented in texi2any

[Gavin Smith]

On Sat, Mar 02, 2019 at 04:33:30PM +0000, Gavin Smith wrote:
> It still needs to be implemented in texi2any, once the syntax is
> finalised. Volunteers are welcome.

I have implemented @subentry in texi2any.  It works for Info, HTML and 
DocBook output, as well as converting back to Texinfo.  (I haven't 
touched the output formats that nobody uses: TexinfoXML, TexinfoSXML, 
IXIN and IXINSXML.)

I haven't done anything extra for @sortas yet.  Before we complicate 
things with @sortas and other commands like @see or @seealso, it might 
be good to see if the current approach with @subentry works for people.