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/whenWithUnit.kt

@@ -0,0 +1,22 @@
+// EXPECTED_REACHABLE_NODES: 1005
+var log = ""
+
+fun foo() {
+    log += "foo()"
+}
+
+fun test(x: Int) = when (x) {
+    in 0..10 -> foo()
+    else -> 55
+}
+
+fun box(): String {
+    val a = test(20)
+    if (a !is Int) return "fail1: $a"
+
+    val b = test(5)
+    if (b !is Unit) return "fail2: $b"
+    if (log != "foo()") return "fail3: $log"
+
+    return "OK"
+}