Allow IOApp to access the default ExecutionContext

Currently it's not possible for users of `IOApp` to use the default
ExecutionContext that backs the ContextShift.

They need to either use `IOApp.WithContext`, or use two unrelated pools, or
override the default `ContextShift`.

This should remove some needless hoop-jumping, I think

Close #748

core/js/src/main/scala/cats/effect/internals/IOAppPlatform.scala

@@ -21,6 +21,7 @@ package internals
 import cats.syntax.all._
 import scala.concurrent.duration._
 import scala.scalajs.js
+import scala.concurrent.ExecutionContext
 
 private[effect] object IOAppPlatform {
   def main(args: Array[String], cs: Eval[ContextShift[IO]], timer: Eval[Timer[IO]])(
@@ -41,8 +42,8 @@ private[effect] object IOAppPlatform {
   /**
    * Sets the exit code with `process.exitCode = code` for runtimes
    * that support it.  This allows a graceful shutdown with a specific
-   * exit code.	
-   *	
+   * exit code.
+   *
    * If the call is not supported and the exit code is not Success,
    * then it is logged.
    *
@@ -82,6 +83,7 @@ private[effect] object IOAppPlatform {
 
   val defaultTimer: Timer[IO] = IOTimer.global
   val defaultContextShift: ContextShift[IO] = IOContextShift.global
+  val defaultExecutionContext: ExecutionContext = ExecutionContext.Implicits.global
 
   private def installHandler(fiber: Fiber[IO, Int]): IO[Unit] = {
     def handler(code: Int) =

core/jvm/src/main/scala/cats/effect/internals/IOAppCompanionPlatform.scala

@@ -83,7 +83,7 @@ private[effect] trait IOAppCompanionPlatform {
      * [[executionContextResource]].  Outside `run`, this context will
      * reject all tasks.
      */
-    final protected def executionContext: ExecutionContext =
+    final override protected def executionContext: ExecutionContext =
       currentContext.get().executionContext
 
     /**

core/jvm/src/main/scala/cats/effect/internals/IOAppPlatform.scala

@@ -18,6 +18,8 @@ package cats
 package effect
 package internals
 
+import scala.concurrent.ExecutionContext
+
 private[effect] object IOAppPlatform {
   def main(args: Array[String], contextShift: Eval[ContextShift[IO]], timer: Eval[Timer[IO]])(
     run: List[String] => IO[ExitCode]
@@ -58,6 +60,9 @@ private[effect] object IOAppPlatform {
   def defaultContextShift: ContextShift[IO] =
     IOContextShift(PoolUtils.ioAppGlobal)
 
+  def defaultExecutionContext: ExecutionContext =
+    PoolUtils.ioAppGlobal
+
   private def installHook(fiber: Fiber[IO, Int]): IO[Unit] =
     IO {
       sys.addShutdownHook {

core/shared/src/main/scala/cats/effect/IOApp.scala

@@ -18,6 +18,7 @@ package cats
 package effect
 
 import cats.effect.internals.{IOAppCompanionPlatform, IOAppPlatform}
+import scala.concurrent.ExecutionContext
 
 /**
  * `App` type that runs a [[cats.effect.IO]].  Shutdown occurs after
@@ -100,6 +101,24 @@ trait IOApp {
    */
   implicit protected def timer: Timer[IO] =
     IOAppPlatform.defaultTimer
+
+  /**
+   * Provides a default `ExecutionContext` for the app.
+   *
+   * The default on top of the JVM is lazily constructed as a fixed
+   * thread pool based on number available of available CPUs (see
+   * `PoolUtils`).
+   *
+   * On top of JavaScript, this will use the standard
+   * [[https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/setTimeout setTimeout]].
+   *
+   * @note This is the same ExecutionContext that backs the default implicit `ContextShift`
+   *
+   * @note To use a different `ExecutionContext`, consider extending `IOApp.WithContext`,
+   *       which will do it more comprehensively.
+   */
+  protected def executionContext: ExecutionContext =
+    IOAppPlatform.defaultExecutionContext
 }
 
 object IOApp extends IOAppCompanionPlatform