Fix warning loading libsing.gd (about SI_number)
Fix (re)setting Singular interpreter error vars

Also moved some more code to calls.cc
Include libsingular.h in libsing.h, so we can use type ring
Move matrix related kernel funcs to src/matrix.cc
Rename SI_ring_of_singobj -> SI_RingOfSingobj
Cleanup indention
Add _SI_MatElm and _SI_SetMatElm