Define constant getter for struct members so we can keep the code DRY

[PaulRBerg]

One of the limitations of structs in Solidity is that they don't have anything equivalent to Rust traits. So testing against some expected shared behavior between multiple different structs is not possible in Solidity (alas).

We could go around this though by defining constant getters for those particular struct members that we're interested in. For example, if we wanted to DRY-ify the renounce function (that is, move it in the ISablierV2 contract), we would have to define only two getters: getStreamSender and isStreamCancelable.

We would of course still have to test these getters on a per-contract basis, but the number of test cases would be just 1, or 2 at worst. The reduce function could thus be moved to the abstract contract ISablierV2 and tested only once.

[PaulRBerg]

An indirect benefit of this would be that it would make it easier for third-party devs and integrating partners to read data from Sablier.

If they are only interested in querying the cancelable state of a stream (which, presumably, would be something of great interest for an NFT lending protocol), an isStreamCancelable would be handy.

[PaulRBerg]

Update: I was wrong when I said that all that is needed to DRY-ify renounce is two getters (getStreamSender and isStreamCancelable).

The renounce function also updates the state of the struct .. and there's no way we can DRY-ify that.