Add mechanism for type coercion in JS

Use it for char boxing/unboxing and unit materialization.
Possible to use for other purposes, for example, to add type checks
to dynamics.

See KT-18793, KT-17915, KT-19081, KT-18216, KT-12970, KT-17014,
KT-13932, KT-13930

js/js.translator/testData/box/coercion/unitSafeCall.kt

@@ -0,0 +1,14 @@
+// EXPECTED_REACHABLE_NODES: 1004
+class C {
+    fun foo() {}
+}
+
+fun box(): String {
+    val a: C? = C()
+    val b: C? = null
+
+    if (a?.foo() != Unit) return "fail1: ${a?.foo()}"
+    if (b?.foo() != null) return "fail2: ${b?.foo()}"
+
+    return "OK"
+}