Add runtime conversion checkers for QuantityPoint

[chiphogg]

I sort of forgot to do this for #110.

[chiphogg]

Updated notes for posterity:

After the latest (and soon-upcoming) changes, this would involve making a new "abstract operation" for adding a magnitude. It would also violate assumptions in the current :overflow_boundary target, namely, that MinGood is always nonpositive and MaxGood is always nonnegative.

We may also have to change the current implementation for QuantityPoint::in, which is based on just delegating to the Quantity operators.

None of this means we shouldn't do this; it's just important information I don't want to have to re-discover when we do this.