[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.

plugins/android-extensions/android-extensions-compiler/src/org/jetbrains/kotlin/android/synthetic/AndroidComponentRegistrar.kt

@@ -40,10 +40,10 @@ import org.jetbrains.kotlin.container.StorageComponentContainer
 import org.jetbrains.kotlin.container.useInstance
 import org.jetbrains.kotlin.descriptors.ModuleDescriptor
 import org.jetbrains.kotlin.extensions.StorageComponentContainerContributor
-import org.jetbrains.kotlin.resolve.TargetPlatform
+import org.jetbrains.kotlin.platform.TargetPlatform
 import org.jetbrains.kotlin.resolve.extensions.SyntheticResolveExtension
+import org.jetbrains.kotlin.platform.jvm.isJvm
 import org.jetbrains.kotlin.resolve.jvm.extensions.PackageFragmentProviderExtension
-import org.jetbrains.kotlin.resolve.jvm.platform.JvmPlatform
 import org.jetbrains.kotlin.utils.decodePluginOptions
 
 object AndroidConfigurationKeys {
@@ -163,9 +163,9 @@ class AndroidComponentRegistrar : ComponentRegistrar {
 
 class AndroidExtensionPropertiesComponentContainerContributor : StorageComponentContainerContributor {
     override fun registerModuleComponents(
-            container: StorageComponentContainer, platform: TargetPlatform, moduleDescriptor: ModuleDescriptor
+        container: StorageComponentContainer, platform: TargetPlatform, moduleDescriptor: ModuleDescriptor
     ) {
-        if (platform != JvmPlatform) return
+        if (!platform.isJvm()) return
 
         container.useInstance(AndroidExtensionPropertiesCallChecker())
         container.useInstance(ParcelableDeclarationChecker())