Implement a "getStreamedAmount" function

[PaulRBerg]

The idea is to take the getWithdrawableAmount function and rename it to getStreamedAmount, and then write up a new getWithdrawableAmount that simply calls getStreamedAmount and subtracts the withdrawn amount.

Benefits:

1. Consistency with the marketing approach. We're marketing ourselves as a streaming protocol, yet we don't have any function that returns the amount streamed so far.
2. Solve the naming issues discussed in Idea to rename "getWithdrawableAmount" to "getRecipientAmount", and "getReturnableAmount" to "getSenderAmount" #227 (at least in part)
3. We would be able to simplify the implementation for the getReturnableAmount function, since we could call getStreamedAmount instead of getWithdrawableAmount, and not subtract the withdrawn amount. This should save a little bit of gas.

Obviously, there's a contract size concern regarding SablierV2Pro. Thus, if we implement the proposal herein, we may have to move the _calculateWithdrawableAmountForMultipleSegments and the _calculateWithdrawableAmountForOneSegment functions in the Helpers library - but, as discussed in #220, I was open to this anyway.

[andreivladbrg]

Like this idea.

This is also how I implemented the functions in the payroll contract.

[PaulRBerg]

Glad to hear that you like it! I implemented this in #266.

[PaulRBerg]

Closed via #266.