Data.Sequence does not check for `Int` overflow

`><`, `replicate`, `replicateA`, `*>`, and maybe other operations are all capable of making sequences that are like the Tardis: bigger on the inside. Any of these (especially when iterated) can easily make a sequence whose size cannot be represented by an `Int`, at which point nothing will work right, and operations will either produce crazy errors or silently give wrong answers. The solution is to check for this in all operations that expand sequences, from `<|` on up. While we're at it, we may also want to switch from `Int` to `Word` size representations internally.