Remove some dead code
Revise initialization of InternalPrimePower

Delay initializing the global state of until it is actually accessed.
This avoids initializing some GMP objects before main() runs.
Get rid of Rational::save

The resulting code is a little bit simpler; the increment/decrement
code is actually more efficient now; and most importantly, this
gets rid of a globally initialized GMP variable.
Get rid of initializeGMP()
Invoke mmInit from Singular's main()

... and not from factory's initCanonicalForm()
Get rid of empty mmInit() functions
Get rid of initCanonicalForm and initializeCharacteristic
Get rid of SI_DONT_HAVE_GLOBAL_VARS
Merge pull request #503 from fingolfin/cleanup-mmInit

Get rid of mmInit()