[Groff] rewrite of chem

[Bernd Warken]

I rewrote `chem' in Perl.  I works without flaws, but it should
be further improved.  So I will need some more time to work
on it.

May I include `chem' into groff?  If yes, on which part of groff
shall `chem' be installed?  I see 2 possibilites:
<groff-top>/contrib/chem for it is a contribution or
<groff-top>/src/preproc/chem because `chem' is a `pic'
preprocessor.

Bernd Warken