fixup! Add Fiber::ExecutionContext::ThreadPool

Author: ysbaddaden

src/fiber/execution_context.cr

@@ -71,7 +71,7 @@ module Fiber::ExecutionContext
   @@default : ExecutionContext?
 
   # :nodoc:
-  def self.thread_pool
+  def self.thread_pool : ThreadPool
     @@thread_pool.not_nil!("expected thread pool to have been setup")
   end