Fix: don't lazy initialize execution contexts linked list [fixup crystal-lang#15350]

It's pointless because we always add the default context anyway. Let's
make the initialization explicit.

Author: ysbaddaden

src/fiber/execution_context.cr

@@ -29,7 +29,7 @@ module Fiber::ExecutionContext
   end
 
   # :nodoc:
-  protected class_getter(execution_contexts) { Thread::LinkedList(ExecutionContext).new }
+  protected class_getter(execution_contexts) = Thread::LinkedList(ExecutionContext).new
 
   # :nodoc:
   property next : ExecutionContext?