David Carlisle's small packages

Many of David Carlisle's more substantial packages stand on their own, or as part of the LaTeX latex-tools set; this set contains: Making dotless 'j' characters for fonts that don't have them; A method for combining the capabilities of longtable and tabularx; An environment for including Plain TeX in LaTeX documents; A jiffy to remove counters from other counters' reset lists (now obsolete as it has been incorporated into the LaTeX format); A jiffy to create 'slashed' characters for physicists.