Add vec::shrink_to_fit

[emberian]

Similar to http://en.cppreference.com/w/cpp/container/vector/shrink_to_fit. I'm not enamored with the name, but there needs to be a way to realloc a vec (without necessarily copying) when you overallocate (using with_capacity).

[thestinger]

I actually have #4960 open about this already.