Add additional logic to function catalog to help with singletons

A registered singleton doesn't have a BeanDefinition, but it might have
a compiled type with enough generic information to pull out the
input and output types.

spring-cloud-function-web/src/test/java/org/springframework/cloud/function/web/SingletonTests.java

@@ -80,7 +80,7 @@ public class SingletonTests {
 				@Override
 				public void postProcessBeanDefinitionRegistry(
 						BeanDefinitionRegistry registry) throws BeansException {
-					// Simulates what happens whem you add a compiled function
+					// Simulates what happens when you add a compiled function
 					RootBeanDefinition beanDefinition = new RootBeanDefinition(MySupplier.class);
 					registry.registerBeanDefinition("words", beanDefinition);
 				}