Properly handle arithmetic exceptions in constant folding

[edfelten]

The compiler should detect all arithmetic exceptions (overflow, underflow, divide by zero, etc) when doing constant folding, and report errors.

Note that this will report errors in some cases where the same operation done at runtime would complete. For example, addition ops don't check for overflow at runtime; they just compute the result mod 2**256. But we think we should report such an overflow as a compile-time error, because the programmer probably doesn't want it to happen.

Later we will return to the issue of how to handle the possibility of runtime arithmetic exceptions.