Bundle "segmentAmounts", "segmentExponents" and "segmentMilestones" into a single array of structs

[PaulRBerg]

It is true that it is not currently possible to copy an in-memory array to storage:

• https://stackoverflow.com/q/49345903/3873510
• https://ethereum.stackexchange.com/q/117658/24693
• Copying of type struct memory[] memory to storage not yet supported ethereum/solidity#3446
• UnimplementedFeatureError with no line indication when copying an array of structs to storage ethereum/solidity#12783

This means that we can't simply assign the segments function argument in a declarative way. If we do that, we would get the following error:

UnimplementedFeatureError: Copying of type struct Segment memory[] memory to storage not yet supported.

However, there is a solution, and that is to iterate over the in-memory array elements and use the push function to push each element, one-by-one, to the storage array:

ProStream storage stream = _streams[streamId];
// other assignments of struct properties
uint256 segmentCount = args.segments.length;
for (uint256 i = 0; i < segmentCount; ++i) {
    stream.segments.push(args.segments[i]);
}

There are several benefits in switching to an array of structs:

1. Fewer function arguments, which helps with alleviating the StackTooDeep problem (re Segment[] array, comptroller, protocol and operator fees, structified parameters, via IR, PRBMath types, major tests refactor #220).
2. Nicer, more encapsulated, less verbose API; segments is more succinct than segmentAmounts, segmentExponents, and segmentMilestones.
3. Fewer errors to check in the checkCreateProArgs function, because we no longer have to ensure that the segment arrays are equal.
   • Consequently, fewer tests to write.