assignment of 'small' to non-'small' should not call 'Util.toInt()'

I made a mistake during the discussion in #6038.

Answering quickly, I didn’t think through the implications of throwing an OverflowException.

Here’s the reasons why not to do that:

1. We don’t generally check for numeric overflow. We generally wrap around silently.
2. Calling a whole method has the potential to suddenly change the performance characteristics of a numeric algorithm, making them much worse, in a way that is not at all predictable to the programmer.
3. The very first usecase for small that I ran into simply broke because of this.

[gavinking]

For everything, I think.

[quintesse]

I guess this can be closed now?