[Wasm] Imporove external interface support

* Support boxing/unboxing when casting to Any
* Support ===, equals, hashCode, toString

* Support adapting String in interop boundary

compiler/testData/codegen/boxWasmJsInterop/jsExport.kt

@@ -9,6 +9,20 @@ fun makeC(x: Int): C = C(x)
 @JsExport
 fun getX(c: C): Int = c.x
 
+@JsExport
+fun getString(s: String): String = "Test string $s";
+
+@JsExport
+fun isEven(x: Int): Boolean = x % 2 == 0
+
+external interface EI
+
+@JsExport
+fun eiAsAny(ei: EI): Any = ei
+
+@JsExport
+fun anyAsEI(any: Any): EI = any as EI
+
 fun box(): String = "OK"
 
 // FILE: jsExport__after.js
@@ -16,4 +30,16 @@ fun box(): String = "OK"
 const c = main.makeC(300);
 if (main.getX(c) !== 300) {
     throw "Fail 1";
+}
+
+if (main.getString("2") !== "Test string 2") {
+    throw "Fail 2";
+}
+
+if (main.isEven(31) !== false || main.isEven(10) !== true) {
+    throw "Fail 3";
+}
+
+if (main.anyAsEI(main.eiAsAny({x:10})).x !== 10) {
+    throw "Fail 4";
 }