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
cross-reference: https://github.com/JuliaLang/julia/pull/8028#discussion_r16518764

@JeffBezanson @StefanKarpinski any thoughts on this observation?