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/nameClashes/propertyAndNativeMethod.kt

@@ -3,7 +3,7 @@
 // FILE: lib.kt
 package lib
 
-external fun bar()
+external fun bar(): Int
 
 val bar = 32