Overhaul SingularIdHdl

It is now both simpler and also more efficient
Add new source file src/singobj.cc
Change Func_SI_CallFunc3 to use SingularIdHdl
Add internal helpers StartPrintCapture / EndPrintCapture
Add Func_SI_CallFunc0, and simplify Func_SI_CallFuncM
Remove reference to  --with-sw from README
Add missing #define for NUM_RAT / DEN_RAT
Improve README some more
Add memfill stress test case by Mohamed
Extend SI_number conversion to Z/pZ to FFE, rationals, bigints