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/lineNumbers/lambdaWithClosure.kt

@@ -4,4 +4,4 @@ fun foo(x: Int): () -> Unit = {
 
 fun bar() = 23
 
-// LINES: 1 1 1 2 2 1 1 1 5 5 5
+// LINES: 1 1 1 2 2 3 3 1 1 1 5 5 5