Exception on overflow in unsigned->signed conversion

[pirapira]

When an unsigned integer is converted to a signed integer of the same size, in half the cases, the original number is not expressible in the target type. This is one form of overflow and it should cause an exception, following the discussion in #796.

[chriseth]

Are you talking about explicit conversions? I think we should not throw an exception for explicit conversions because otherwise there is no way to actually perform explicit overflow.

There have been suggestions to provide two types of explicit conversions with different error behaviour, but there is no proposal on syntax yet.

[pirapira]

I'm trying to enumerate overflows in arithmetics.

[pirapira]

Delegated back to #796.