Enable lazier initialization of Dispatchers.IO (#4166)

Currently, any reference to Dispatchers class, including many calls
to Dispatchers.Default, force Dispatchers.IO to be initialized. This
incurs several system property reads and other expensive operations
even if Dispatchers.IO is never used.

This PR changes this by delaying Dispatchers.IO initialization
until it's first accessed.

Author: metalhead8816

kotlinx-coroutines-core/jvm/src/Dispatchers.kt

@@ -62,7 +62,7 @@ public actual object Dispatchers {
      * during operations over IO dispatcher.
      */
     @JvmStatic
-    public val IO: CoroutineDispatcher = DefaultIoScheduler
+    public val IO: CoroutineDispatcher get() = DefaultIoScheduler
 
     /**
      * Shuts down built-in dispatchers, such as [Default] and [IO],