Reference count as a first class type

[gdamore]

We use atomics for reference counts -- especially e.g. on messages were we need to know if the message is held exclusively or not. Testing this currently is done using an atomic. However, in the case where the message is not shared, an atomic operation or barrier is not needed. As this sometimes occurs in hot code paths, it might be nice to make reference counts a first class data type, and support just a local variable read if the reference count is <= 1.