[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/ide-common/src/org/jetbrains/kotlin/caches/resolve/KotlinCacheService.kt

@@ -22,7 +22,7 @@ import com.intellij.psi.PsiFile
 import org.jetbrains.kotlin.analyzer.ModuleInfo
 import org.jetbrains.kotlin.idea.resolve.ResolutionFacade
 import org.jetbrains.kotlin.psi.KtElement
-import org.jetbrains.kotlin.resolve.TargetPlatform
+import org.jetbrains.kotlin.platform.TargetPlatform
 import org.jetbrains.kotlin.resolve.diagnostics.KotlinSuppressCache
 
 interface KotlinCacheService {