This commit works by adding a SmallFunction class that behaves
like std::function, but allocates its data inline rather than through
a separate allocation.  It probably could have also worked by taking
a custom allocator and using the new RegionAllocator.

It adds a bit more restrictions than std::function does (the types
caught by the closure have to be more "trivial" than std::function
supports), so some of this change is marking some types as trivial,
or copying data into a trivial format from things that aren't (ex SmallVector).