Add a few list helper functions

[ag-eitilt]

There's been a few times I and @maniltongya have lamented not having some less-obvious List helpers we'd reach for in Haskell; this adds the two we had talked about previously, and I quickly scanned through Data.List but didn't see anything else immediately relevant -- feel free to suggest others, though, if I did miss something.

Additionally, this implements the note that we'd eventually add single for similar limited benefit as prepend (i.e. not having to write a parenthetical underscore-lambda when we want to pass it to some other function). I deliberately haven't brought over the similarly-notated lookup since it's not the type signature or semantics people would expect from that name, people should probably use a Map if they need to make a lot of mlookup calls, and there's enough variation in whether they'd want just the value or the key and the value (since the predicate might be written to match multiple keys) that I didn't feel a single helper was helpful when we already have find.