Fix Int8(-128) // Int8(-128) (#51944)
Fixes #51731 and #51730
Also fixes one of the previously broken tests:
```julia
@test_broken Rational{Int64}(UInt(1), typemin(Int32)) == Int64(1) // Int64(typemin(Int32))
```
This PR ensures the `Rational{T}` constructor with concrete `T` will
only throw if the final numerator and denominator cannot be represented
by `T`, or are both zero.
If the `T` in `Rational{T}` is not concrete, this PR tries to ensure the
numerator and denominator are promoted to the same type.
This means `-1*Rational{Integer}(-1, 0x01) == 1` doesn't throw now. A
side effect of this is that `Rational{Integer}(Int8(-1), 0x01)` now
throws.
Also, related to
<https://github.com/JuliaLang/julia/pull/25702#issuecomment-359821951>,
now `divgcd` doesn't change the types of its inputs and can handle
`typemin`, but it still throws a `DivideError` if both inputs are zero.
