Auto merge of #43248 - llogiq:num-try-from, r=nagisa
improve the TryFrom implementations
This removes the need for a 128 bit storage by making use of the fact that there can be either no over/underflow, either one or both, and each time the target type suffices to hold the limit for comparison. This also means that the implementation will work in targets without 128bit support (unless it's for 128bit types, of course).
The downside is that the code looks a bit more complex.