Add refillable streams

[TomAFrench]

Using essentially the same method for withdrawals but in reverse it's possible to refill a stream without needing to create a new one by just pushing out the stream's finish time.

There are some issues that crop up if the stream has already finished:

1. If the stream is fully withdrawn, the current value:time ratio can't be maintained while adding funds
2. If the stream isn't fully withdrawn but the end time has passed, refilling the stream potentially makes a large chuck of the extra money instantly available to the recipient unexpectedly.

For 1, this can be resolved by simply overwriting the stream's information with a new one. Likely this raises new issues (such as difficult to get a history of past streams) for little benefit.

For 2, lastWithdrawnTime could be brought forward to compensate for this but it brings in additional complexity again for not much benefit.

It's probably best to just require the creation of a new stream in both cases.