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/receiverSmartCast.kt

@@ -0,0 +1,32 @@
+// EXPECTED_REACHABLE_NODES: 997
+fun foo(x: Any): String {
+    return when (x) {
+        is Char -> "char: ${x.toInt()}"
+        else -> "other: $x"
+    }
+}
+
+fun bar(x: Any): String {
+    return when (x) {
+        is Char -> "char: ${x.baz()}"
+        else -> "other: $x"
+    }
+}
+
+fun Char.baz(): Boolean = jsTypeOf(asDynamic()) == "number"
+
+fun box(): String {
+    val a = foo('0')
+    if (a != "char: 48") return "fail1: $a"
+
+    val b = foo(23)
+    if (b != "other: 23") return "fail2: $b"
+
+    val c = bar('0')
+    if (c != "char: true") return "fail3: $c"
+
+    val d = bar(23)
+    if (d != "other: 23") return "fail4: $d"
+
+    return "OK"
+}