Add rounding up support for 1 wei claims

[Willyham]

Fixes #126

Comment which I wanted to put into the code but it's not worth the space. There is a very unlikely but quite interesting edge case here. We think it's worth the tradeoff to keep clearer accounting for the stAVAX rounding in the more 'normal' case.

        // This technically means that we are giving people slightly more AVAX in exchange for their stAVAX if they
        // claim 1 wei's worth. In theory you could claim everything in 1 wei increments to earn more, but gas makes
        // this clearly infeasible. However, the end result of a few of these round-ups is that there will not be
        // enough stAVAX in the protocol for the very last unstaker. In practise this will not happen because we will
        // stake some AVAX at deploy which we'll never remove.