Fix delimiter remainder and add capability to split on other delimiters

[GeorgeLyon]

Slightly tweaked version of: https://github.com/GeorgeLyon/Shwift/pull/25/files

This PR also fixes an issue with Shwift tests I accidentally introduced in 3.0, and some other light cleanup.

[GeorgeLyon]

@wti Sorry to hijack your PR, but I don't know how to push commits to not-my PR. This should add the requested delimiter functionality to Shwift.

I removed the Script changes and tests (I wasn't too fond of the fake-Decodable thing, and having the functionality exercised in Shwift without any of the Script machinery is nice)... when I have some time I'll try to add the AsyncSequence variant of pipe

I also added Segments as a sibling type to Line so that we can both not change the current API and have a type that is correctly named ("Lines" isn't a good name for things not separated by "\n"). Hopefully soon we'll be able to use some AsyncSequence<Element, any Error>.