[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/copyright/apache.xml

@@ -0,0 +1,7 @@
+<component name="CopyrightManager">
+  <copyright>
+    <option name="allowReplaceRegexp" value="JetBrains" />
+    <option name="notice" value="Copyright 2010-&amp;#36;today.year JetBrains s.r.o. and Kotlin Programming Language contributors.&#10;Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file." />
+    <option name="myName" value="apache" />
+  </copyright>
+</component>