refactor: voiding

[PaulRBerg]

Addresses #23.

This PR does three things:

1. Refactors the streamedAmountOf function to void the stream if there is a calculation error and the streamed amount exceeds the deposit amount. If this bug occurs (which should never happen, but still):
   • In the linear streams and in the single-segment dynamic streams, the withdrawn amount is considered the streamed amount, which in effect freezes the stream and lets the sender cancel and recover their funds.
   • In the multi-segment dynamic streams, the segment streamed amount is considered zero, which in effect voids the segment.
2. Removes the unchecked blocks in cancel, _withdraw, returnableAmountOf, and withdrawableAmountOf.
3. Removes the now-unneeded assertion amounts.deposit >= amounts.withdrawn in the _withdraw function

[PaulRBerg]

Merging now. Coverage decreased, but that's expected - we added code that we don't know how to test (because we think it should never happen).