Get rid of mmInit()

We discussed a bit about mmInit in Aachen. Back then I learned that the reason for its existence was that a few global variables involved GMP values. This in turn made it necessary to initialize the GMP memory management facilities *before* main() is executed. This in turn resulted in various other annoying hacks (e.g. the SI_DONT_HAVE_GLOBAL_VARS #define) in the code.

I took a look with a debugger to figure out which vars are the culprits here. It turns out that only two places use GMP values in a globally constructed object. This commit series first does some simple code cleanup, then gets rid of both uses. This in turn allows removal of most of the empty mmInit() implementation; now only the mmInit() in tesths.cc remains, and the first thing Singular's main() now does is to call it.

This enables further cleanup, in particular one can get rid of SI_DONT_HAVE_GLOBAL_VARS.