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/destructuringToUnit.kt

@@ -0,0 +1,20 @@
+// EXPECTED_REACHABLE_NODES: 1006
+var log = ""
+
+class A {
+    operator fun component1() {
+        log += "A.component1()"
+    }
+
+    operator fun component2() = 23
+}
+
+fun box(): String {
+    val (x: Any, y) = A()
+
+    if (x != Unit) return "fail1: $x"
+    if (y != 23) return "fail2: $y"
+    if (log != "A.component1()") return "fail3: $log"
+
+    return "OK"
+}