Remove isCanceled and isStream from Stream struct

[smol-ninja]

This PR removes isCanceled and isStream fields from the Stream struct. Functions isCanceled(uint256 streamId) and function isStream(uint256 streamId) are modified as the following:

function isCanceled(uint256 streamId) public view override notNull(streamId) returns (bool result) {
    result = _streams[streamId].ratePerSecond == 0 && _streams[streamId].sender != address(0);
}

function isStream(uint256 streamId) public view returns (bool result) {
    result = _streams[streamId].sender != address(0);
}

Results

Function name	Max (previous)	Max (new)	Gas efficiency
adjustRatePerSecond	21008	19022	9.45%
cancel	73332	68310	6.85%
create	105213	104501	0.68%
createAndDeposit	105012	104443	0.54%
deposit	49295	49309	-0.03%
refundFromStream	29469	29483	-0.05%
restartStream	34788	32441	6.75%
restartStreamAndDeposit	37156	35919	3.33%
withdraw	49950	47964	3.98%

[andreivladbrg]

Thank you @smol-ninja for the PR.

I consider relying on specific boolean variables, that are toggled only upon certain actions, to be a safer method, despite the gas implications.

Also, the current design is similar to the one in lockup contracts - it is a good thing to have similar design patterns

Are you ok with closing this PR?

[smol-ninja]

it is a good thing to have similar design patterns

Agree on this point. Closing this.