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/loopOverUnits.kt

@@ -0,0 +1,25 @@
+// EXPECTED_REACHABLE_NODES: 1009
+class A {
+    operator fun iterator() = B()
+}
+
+class B() {
+    private var count = 0
+
+    operator fun next() {
+        count++
+    }
+
+    operator fun hasNext() = count < 5
+}
+
+fun box(): String {
+    var i = 0
+    for (x: Any in A()) {
+        if (x != Unit) return "fail1: $x"
+        i++
+    }
+    if (i != 5) return "fail2: $i"
+
+    return "OK"
+}