Find or create overflow-proof safe arithmetic library

[cwgoes]

Ref #694

Put this in tmlibs and make sure we use it.

[rigelrozanski]

What does this entail that errors are returned instead of panics? Do we really need this if we're catching all panics within SDK already?

[cwgoes]

Panics are fine - probably preferable - they just need to happen instead of overflows.

[rigelrozanski]

......... does go arithmetic not panic on overflows? https://medium.com/@griffinish/integer-overflow-in-golang-9e13e274c8a5
Guess not>
This article suggests this library in it https://github.com/JohnCGriffin/overflow

[rigelrozanski]

but again we should probably not import but add it ourselves to tmlibs or whatever

[cwgoes]

Less necessary because of #977, still a good idea where we aren't using big.Int though.

[ebuchman]

Do we still want to do this? Or just push everything through big.Int?

[rigelrozanski]

• for everything through big.Int

[cwgoes]

I think we can close this for now. If end users want to use non-big.Int types and demand later emerges for such a library, we can reconsider.