Explicitly declare stability levels of declarations in kotlinx.cinterop package

* @ExperimentalForeignApi for all declarations that operate on
    unmanaged memory (i.e. pointers and references)
* @BetaInteropApi for the rest of the interoperability declarations,
    namely Swift/CInterop-specific interfaces and convenience-functions

### Implementation details

* Some typealiases are not marked explicitly because it crashes the compiler,
    yet their experimentality is properly propagated
* License header is adjusted where it previously had been existing
* Deprecated with ERROR interop declarations that are deprecated for more than
    two years are removed
* WASM target interop declarations are deprecated
* Deliberately make Boolean.toByte and Byte.toBoolean foreign-experimental to scare
    people away

^KT-57728 fixed

Merge-request: KT-MR-9788
Merged-by: Vsevolod Tolstopyatov <qwwdfsad@gmail.com>

kotlin-native/backend.native/tests/runtime/basic/cleaner_basic.kt

@@ -2,7 +2,8 @@
  * Copyright 2010-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the LICENSE file.
  */
-@file:OptIn(ExperimentalStdlibApi::class, FreezingIsDeprecated::class, kotlin.native.runtime.NativeRuntimeApi::class)
+@file:OptIn(ExperimentalStdlibApi::class, FreezingIsDeprecated::class,
+        kotlin.native.runtime.NativeRuntimeApi::class, kotlinx.cinterop.ExperimentalForeignApi::class)
 
 package runtime.basic.cleaner_basic