Fix: Crystal::SpinLock doesn't need to be allocated in the HEAP

The abstraction is a mere abstraction over an atomic integer and the
object itself are only ever used internally of other objects, with the
exception of Channel where the code explicitely accesses the ivar
directly (thus not making copies).

We can avoid a HEAP allocation everywhere we use them (i.e. in lots of
places).

src/crystal/spin_lock.cr

@@ -1,5 +1,5 @@
 # :nodoc:
-class Crystal::SpinLock
+struct Crystal::SpinLock
   private UNLOCKED = 0
   private LOCKED   = 1