[Platform API] Introduce fundamental abstraction of Platform

This is a large commit, which introduces general API for working with
abstraction of Platform.

- Add new abstraction to 'core' - SimplePlatform - which represents
exactly one platform
  - Clients are strongly prohibited to create instances of SimplePlatform
  by hand, instead, corresponding *Platforms abstraction should be used
  (e.g. JvmPlatforms, JsPlatforms, KonanPlatforms)

- Move TargetPlatform to 'core', it represents now a collection of
SimplePlatforms
  - Clients are strongly encouraged to use TargetPlatform
    (not SimplePlatform) in API, to enforce checks for multiplatform

- Provide a helper-extensions to work with TargetPlatform
(in particular, for getting a specific component platform)

- Remove MultiTargetPlatform in favour of TargetPlatform
  - Notably, this commit leaves another widely used duplicated abstraction,
    namely, IdePlatform. For the sake sanity, removal of IdePlatform is
    extracted in the separate commit.

idea/idea-android/src/org/jetbrains/kotlin/android/configure/KotlinAndroidGradleModuleConfigurator.kt

@@ -17,14 +17,14 @@ import org.jetbrains.kotlin.idea.configuration.getBuildSystemType
 import org.jetbrains.kotlin.idea.util.projectStructure.version
 import org.jetbrains.kotlin.idea.versions.MAVEN_STDLIB_ID_JDK7
 import org.jetbrains.kotlin.idea.versions.hasJreSpecificRuntime
-import org.jetbrains.kotlin.resolve.TargetPlatform
-import org.jetbrains.kotlin.resolve.jvm.platform.JvmPlatform
+import org.jetbrains.kotlin.platform.jvm.JvmPlatforms
+import org.jetbrains.kotlin.platform.TargetPlatform
 
 class KotlinAndroidGradleModuleConfigurator internal constructor() : KotlinWithGradleConfigurator() {
 
     override val name: String = NAME
 
-    override val targetPlatform: TargetPlatform = JvmPlatform
+    override val targetPlatform: TargetPlatform = JvmPlatforms.defaultJvmPlatform
 
     override val presentableText: String = "Android with Gradle"