halfagg: Remove fixed limit

[real-or-random]

This is one way to deal with the limit. My thinking is that we shouldn't be responsible for the limit.

The drawback of this approach is that it removes some tests. The rust spec is still correct in the sense that you can't possibly have two Seqs whose combined size is larger than usize::MAX because you won't have enough memory. But that's rather implicit, and it may not be true for implementations using other data types, or in other languages etc. Implementations that want to enforce a limit will have to figure out how to do it, because the spec won't help them.