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/lambdaParameters.kt

@@ -0,0 +1,22 @@
+// EXPECTED_REACHABLE_NODES: 997
+// CHECK_NOT_CALLED_IN_SCOPE: function=toBoxedChar scope=box$lambda
+// CHECK_CALLED_IN_SCOPE: function=unboxChar scope=box$lambda
+// CHECK_CALLED_IN_SCOPE: function=toBoxedChar scope=box
+// CHECK_NOT_CALLED_IN_SCOPE: function=unboxChar scope=box
+
+fun <T> bar(x: T, y: (T) -> Boolean): Boolean = y(x) && jsTypeOf(x.asDynamic()) != "number"
+
+fun typeOf(x: dynamic) = js("typeof x")
+
+fun box(): String {
+    val f = { x: Char ->
+        val a: Char = x
+        val b: Any = x
+        typeOf(a) == "number" && typeOf(b) == "object"
+    }
+
+    if (!f('Q')) return "fail1"
+    if (!bar('W', f)) return "fail2"
+
+    return "OK"
+}