Rename core::vec::last_total to core::vec::last

[brson]

The current vec::last returns an option if there are no elements to return, while all the related functions fail. vec::last should just fail as well when the vec is empty

[catamorphism]

Since the precondition has been removed from last_total, the name is wrong anyway, as is the associated comment.

[catamorphism]

I'm renaming last_total to last_unsafe. IMO the other functions should return an option too rather than failing, and variants that fail should have "unsafe" in the name. (Using a word other than "unsafe" would be ok with me, I just can't think of anything better.)