Rounding issue on small amounts

[rafaelugolini]

While investigating ousd_analytics, I found a small issue with small numbers:

The user deposited E 0.001 and got OETH 0.000999999999999999
https://etherscan.io/tx/0x9b005270a8ea48d6ea28f9f72550ba814dd579d73c81596eeaa82296ec5544b3

Funny enough, etherscan rounds it up.

[DanielVF]

DanielVF: Which is to say that mints of amounts below 1e18 always return 1 less
[8:11 PM]DanielVF: I think this also applies to transfers
[8:12 PM]DanielVF: This does not apply when transfering to a non-rebasing contract
[8:13 PM]DanielVF: Yes, it's rounding related
[8:13 PM]DanielVF: We've just always done our tests / play testing on OUSD with amounts > 1e18
[8:17 PM]DanielVF: OUSD is correctly recording the amount in their internal balance
[8:17 PM]DanielVF: But the balanceOf calculations are having the rounding error
[8:18 PM]DanielVF: But I guess it's not just mint related, since that's also the same calculations used for transfers + balances