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/char/charEquals.kt

@@ -9,5 +9,15 @@ fun box(): String {
     assertEquals(false, 'A'== 'B')
     assertEquals(false, ('A' as Any) == (65 as Any))
 
+    assertTrue(bar('Q'))
+    assertFalse(bar('W'))
+
+    assertTrue(baz('Q'))
+    assertFalse(baz('W'))
+
     return "OK"
-}
+}
+
+fun bar(x: Char) = x.equals('Q')
+
+fun baz(x: Any) = x.equals('Q')