Stack-allocated collections

[taylorswift]

@stuchlej

I believe this class allows you to access the same optimisations as most of the Swift native collections use. It is widely used by the swift-collections repo (for example).

alloca and ManagedBuffer are different things nowadays, the easiest way to get a stack allocation now is to just call withUnsafeTemporaryAllocation(of:capacity:_:).