Add methods for checked additions/subtractions of `Amount`s

[james-chf]

Amount::spend and Amount::receive panic if there is an overflow/underflow. We want methods as well to do checked additions or subtractions safely, e.g. we may want a VP to reject a transaction in the normal way if it would overflow rather than panicking.

[sug0]

slight note: the referred methods only panic in debug builds. in release builds, no panic will occur, which is quite bad, because we may accept/reject a VP that would have otherwise been rejected/accepted.

[cwgoes]

We should always be using checked arithmetic. @bengtlofgren is this maybe a quick change you could do?

[james-chf]

NB: there is now already checked_sub, just need a checked_add method (+ some test coverage) https://github.com/anoma/namada/blob/76169dcdaf15911ac2f69dec78604e18c4c5b259/core/src/types/token.rs#L74-L79