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
@vtjnash: Ok, I don't understand this. But this maybe because I do not understand the difference between `Int32` and `Type{Int32}`. Is the difference described in the manual?