Make codebase less partial

[ulysses4ever]

There are many heads and tails across the code, and since GHC 9.8 they all produce warnings by default. I had to switch our project file from -Werror to -Werror=x-no-partial, sadly.

It'd be a good spring-cleaning/newcomer task to clean these a bit. Worst case you can replace head with headDef with an erroring term that explains the context. But better yet, many of these have to do with how the core datatypes are defined in Gibbon: they make extensive use of the list type, while in some cases those lists can't be empty, and so could be replaced with NonEmpty.