Silent overflow in converting BigInt to Int128

[jiahao]

As uncovered in #6163:

julia> convert(Int64, BigInt("172293789645374271893931539237738556000"))
ERROR: InexactError()
 in convert at gmp.jl:108

julia> convert(Int128, BigInt("172293789645374271893931539237738556000"))
167988577275564191569443068194029655456

[jiahao]

As it so happens, this number is Uint128-representable, thus causing the silent overflow in this method:

julia> typemax(Uint128) > BigInt("172293789645374271893931539237738556000") > typemax(Int128)
true

[jiahao]

julia> convert(Uint128, BigInt("172293789645374271893931539237738556000"))
0x819e93add1572fe4413993ece0f2aa60

The copysign is interfering with the expected wraparound behavior:

julia> convert(Int16, 0xffff)
-1

[JeffBezanson]

All of our integer conversions are unchecked (#5413). I'm not sure why that one conversion has an InexactError but it might have to do with the behavior of GMP's API here.