[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/nullableExternRefs.kt

@@ -0,0 +1,65 @@
+// TARGET_BACKEND: WASM
+// WITH_RUNTIME
+
+// FILE: externals.js
+
+function roundTrip(x) { return x; }
+
+// FILE: externals.kt
+
+external fun roundTrip(x: EI?): EI?
+
+fun assertTrue(x: Boolean) {
+    if (!x) error("assertTrue fail")
+}
+
+fun assertFalse(x: Boolean) {
+    if (x) error("assertFalse fail")
+}
+
+external interface EI
+
+@JsFun("() => null")
+external fun getNull(): EI?
+
+@JsFun("() => undefined")
+external fun getUndefined(): EI?
+
+@JsFun("(ref) => ref === null")
+external fun isJsNull(ref: EI?): Boolean
+
+@JsFun("(ref) => ref === undefined")
+external fun isJsUndefined(ref: EI?): Boolean
+
+@JsFun("() => null")
+external fun getJsNullAsNonNullable(): EI
+
+@JsFun("() => undefined")
+external fun getJsUndefinedAsNonNullable(): EI
+
+fun box(): String {
+    val jsNull = getNull()
+    val jsUndefined = getUndefined()
+
+    assertTrue(jsNull == null)
+    assertTrue((jsNull as Any?) == null)
+    assertTrue((jsNull as Any?) === null)
+    assertTrue(jsUndefined == null)
+
+    assertTrue(isJsNull(null))
+    assertTrue(isJsNull(null as EI?))
+    assertTrue(isJsNull(null as? EI?))
+    assertTrue(isJsNull(roundTrip(null)))
+    assertTrue(isJsNull(jsNull))
+
+    assertFalse(isJsUndefined(null))
+    assertTrue(isJsUndefined(jsUndefined))
+
+    // TODO: Should these fail?
+    val n2 = getJsNullAsNonNullable()
+    val ud2 = getJsUndefinedAsNonNullable()
+    assertTrue(isJsNull(n2))
+    assertTrue(isJsUndefined(ud2))
+
+    return "OK"
+}