Log Message:
tex-mathtools: update to 1.28a

* added note about \eqref and showonlyrefs
* fixed typo in note about strange 3-arg \bracket command example
* fixed typo in section 3.4.4 (strange \hphantom{g})
* removed robustifying \(\)\[\], now done in the LaTeX kernel
* added \xlongrightarrow and \xlongleftarrow
* added \MakeAboxedCommand to generate \Aboxed like macros
* fixed year typo in \ProvidesPackage