[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.

compiler/cli/cli-common/build.gradle.kts

@@ -8,6 +8,7 @@ dependencies {
     compile(project(":core:util.runtime"))
     compile(project(":compiler:frontend"))
     compile(project(":compiler:frontend.java"))
+    compile(project(":js:js.frontend"))
     compileOnly(project(":kotlin-reflect-api"))
     compileOnly(intellijCoreDep()) { includeJars("intellij-core") }
     compileOnly(intellijDep()) { includeIntellijCoreJarDependencies(project) }