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/charBoxing.kt

@@ -1,20 +0,0 @@
-var log = ""
-
-inline fun String.foo(a: String): Int {
-    log += "foo1"
-    return asDynamic().indexOf(a)
-}
-
-inline fun String.foo(a: Char): Int {
-    log += "foo2"
-    return indexOf(a.toString())
-}
-
-fun bar(a: String, b: Char, x: Int) {
-    log += if (x > 0)
-        23
-    else
-        a.foo(b)
-}
-
-// LINES: 6 4 4 5 5 8 8 8 8 8 8 8 11 9 9 10 10 8 8 8 8 18 14 14 14 14 14 15 17 17 9 9 14 10 17 10 14 14 * 1 * 1

js/js.translator/testData/lineNumbers/destructuring.kt

@@ -27,4 +27,4 @@ fun bar(f: (Pair<String, String>) -> Unit) {
 }
 
 
-// LINES: 22 16 16 17 18 20 20 21 21 23 9 9 9 9 4 9 9 9 5 5 6 7 11 11 12 12 15 15 27 26 26 * 1 * 1
+// LINES: 22 16 16 17 18 20 20 21 21 22 22 23 9 9 9 9 4 9 9 9 5 5 6 7 11 11 12 12 15 15 27 26 26 * 1 * 1

js/js.translator/testData/lineNumbers/destructuringInline.kt

@@ -32,4 +32,4 @@ inline operator fun P.component1() = a
 
 inline operator fun P.component2() = b
 
-// LINES: 22 17 17 31 18 18 33 20 20 21 21 23 9 9 9 9 4 9 9 9 6 6 31 7 7 33 11 11 12 12 15 15 27 26 26 29 29 29 * 31 31 31 33 33 33 * 1 * 1
+// LINES: 22 17 17 31 18 18 33 20 20 21 21 22 22 23 9 9 9 9 4 9 9 9 6 6 31 7 7 33 11 11 12 12 15 15 27 26 26 29 29 29 * 31 31 31 33 33 33 * 1 * 1

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