[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-idea/src/org/jetbrains/kotlin/android/synthetic/idea/ExperimentalUtils.kt

@@ -29,7 +29,7 @@ import org.jetbrains.kotlin.android.synthetic.AndroidComponentRegistrar.Companio
 import org.jetbrains.kotlin.compiler.plugin.AbstractCliOption
 import org.jetbrains.kotlin.idea.core.unwrapModuleSourceInfo
 import org.jetbrains.kotlin.idea.facet.KotlinFacet
-import org.jetbrains.kotlin.resolve.jvm.platform.JvmPlatform
+import org.jetbrains.kotlin.platform.jvm.isJvm
 
 private val ANNOTATION_OPTION_PREFIX = "plugin:$ANDROID_COMPILER_PLUGIN_ID:"
 
@@ -53,7 +53,7 @@ private fun isTestMode(module: Module): Boolean {
 internal val Module.androidExtensionsIsEnabled: Boolean
     get() = isTestMode(this) || getOptionValueInFacet(ENABLED_OPTION) == "true"
 
-internal fun ModuleInfo.findAndroidModuleInfo() = unwrapModuleSourceInfo()?.takeIf { it.platform is JvmPlatform }
+internal fun ModuleInfo.findAndroidModuleInfo() = unwrapModuleSourceInfo()?.takeIf { it.platform.isJvm() }
 
 internal val ModuleInfo.androidExtensionsIsEnabled: Boolean
     get() {