only use 'catalan' for Catalan's constant, not 'G'

This constant is way too rarely used (in comparison with `e`) to give it a single-letter name `G` in the global namespace.

Not only is `G` used for other [common constants](http://en.wikipedia.org/wiki/Gravitational_constant), but people don't even agree on `G` for Catalan's constant.  MathWorld uses `K`, Wikipedia uses `G`, and Mathematica uses `C`.  (Maple uses `Catalan`.)