Introduce Kotlin functional bean definition DSL

As a follow-up of the ApplicationContext Kotlin extensions, close to
the Kotlin functional WebFlux DSL and partially inspired of the
Groovy/Scala bean configuration DSL, this commit introduces a
lightweight Kotlin DSL for functional bean declaration.

It allows declaring beans as following:

beans {
	bean<Foo>()
	profile("bar") {
		bean<Bar>("bar", scope = Scope.PROTOTYPE)
	}
	environment({ it.activeProfiles.contains("baz") }) {
		bean { Baz(it.ref()) }
		bean { Baz(it.ref("bar")) }
	}
}

Advantages compared to Regular ApplicationContext API are:
 - No exposure of low-level ApplicationContext API
 - Focused DSL easier to read, but also easier to write with a fewer
   entries in the auto-complete
 - Declarative syntax instead of functions with verbs like registerBeans
   while still allowing programmatic registration of beans if needed
 - Such DSL is idiomatic in Kotlin
 - No need to have an ApplicationContext instance to write how you
   register your beans since beans { } DSL is conceptually a
   Consumer<GenericApplicationContext>

This DSL effectively replaces ApplicationContext Kotlin extensions as
the recommended way to register beans in a functional way with Kotlin.

Issue: SPR-15755

spring-context/src/test/kotlin/org/springframework/context/support/BeanDefinitionDslTests.kt

@@ -0,0 +1,101 @@
+/*
+ * Copyright 2002-2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.context.support
+
+import org.junit.Assert.*
+import org.junit.Test
+import org.springframework.beans.factory.NoSuchBeanDefinitionException
+import org.springframework.beans.factory.getBean
+import org.springframework.context.support.BeanDefinitionDsl.*
+
+class BeanDefinitionDslTests {
+	
+	@Test
+	fun `Declare beans with the functional Kotlin DSL`() {
+		val beans = beans { 
+			bean<Foo>()
+			bean<Bar>("bar", scope = Scope.PROTOTYPE)
+			bean { Baz(it.ref<Bar>()) }
+			bean { Baz(it.ref("bar")) }
+		}
+
+		val context = GenericApplicationContext()
+		beans.invoke(context)
+		context.refresh()
+		
+		assertNotNull(context.getBean<Foo>())
+		assertNotNull(context.getBean<Bar>("bar"))
+		assertTrue(context.isPrototype("bar"))
+		assertNotNull(context.getBean<Baz>())
+	}
+
+	@Test
+	fun `Declare beans using profile condition with the functional Kotlin DSL`() {
+		val beans = beans {
+			bean<Foo>()
+			bean<Bar>("bar")
+			profile("baz") {
+				profile("pp") {
+					bean<Foo>()
+				}
+				bean { Baz(it.ref<Bar>()) }
+				bean { Baz(it.ref("bar")) }
+			}
+		}
+
+		val context = GenericApplicationContext()
+		beans.invoke(context)
+		context.refresh()
+
+		assertNotNull(context.getBean<Foo>())
+		assertNotNull(context.getBean<Bar>("bar"))
+		try { 
+			context.getBean<Baz>()
+			fail("Expect NoSuchBeanDefinitionException to be thrown")
+		}
+		catch(ex: NoSuchBeanDefinitionException) { null }
+	}
+
+	@Test
+	fun `Declare beans using environment condition with the functional Kotlin DSL`() {
+		val beans = beans {
+			bean<Foo>()
+			bean<Bar>("bar")
+			environment({it.activeProfiles.contains("baz")}) {
+				bean { Baz(it.ref()) }
+				bean { Baz(it.ref("bar")) }
+			}
+		}
+
+		val context = GenericApplicationContext()
+		beans.invoke(context)
+		context.refresh()
+
+		assertNotNull(context.getBean<Foo>())
+		assertNotNull(context.getBean<Bar>("bar"))
+		try {
+			context.getBean<Baz>()
+			fail("Expect NoSuchBeanDefinitionException to be thrown")
+		}
+		catch(ex: NoSuchBeanDefinitionException) { null }
+	}
+	
+}
+
+class Foo
+class Bar
+class Baz(val bar: Bar)