Use SmallVec<[T; 1]> where appropriate

[kazimuth]

This is a minor perf thing:

SmallVec<[T;N]> is a type that stores can store up to N values inline before spilling to the heap, but otherwise behaves exactly the same as Vec. We should use SmallVec in types that there will be many copies of, for example Unit, to avoid extra heap allocations.