Add methods for checked additions/subtractions of Amounts

[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)

namada/core/src/types/token.rs

Lines 74 to 79 in 76169dc

	/// Checked subtraction
	pub fn checked_sub(&self, amount: Amount) -> Option<Self> {
	self.micro
	.checked_sub(amount.micro)
	.map(|result| Self { micro: result })
	}