RFC: Preserve type on integer arithmetic

This is my try to make `Int8 + Int8 = Int8`. It would fix #3759 which now has the 0.4-projects tag indicating that there is hope that this gets accepted :-)

I had to fix some tests and the unicode test still fails with

    exception on 2: ERROR: test failed: utf8(u16) == u8
I think I would call these `SignedUpto128`, etc. for better readability.