Return the bound instance when using Container::instance

[adamwathan]

In many of my feature tests I need to replace something in the container with a test double, but keep a reference to the test double to use in my assertions.

Currently I'm declaring the variable at the same time I pass it into the container:

$this->app->instance(CreditCardGateway::class, $creditCardGateway = new FakeCreditCardGateway);
$this->app->instance(WebhookDispatcher::class, $webhookDispatcher = new FakeWebhookDispatcher);

I think this is sort of grim, so this PR lets me rewrite that like this:

$creditCardGateway = $this->app->instance(CreditCardGateway::class, new FakeCreditCardGateway);
$webhookDispatcher = $this->app->instance(WebhookDispatcher::class, new FakeWebhookDispatcher);

I think it makes it a lot easier to see where the variable is coming from at a glance 👍