Inherit JSFunction from JSClosure

There is no reason not to make JSClosure to be compatible with
JSFunction. We can treat JSClosure as a JSFunction and call it
from not only JavaScript but also Swift.

IntegrationTests/TestSuites/Sources/PrimaryTests/UnitTestUtils.swift

@@ -14,6 +14,7 @@ func test(_ name: String, testBlock: () throws -> Void) throws {
         print(error)
         throw error
     }
+    print("✅ \(name)")
 }
 
 struct MessageError: Error {

IntegrationTests/TestSuites/Sources/PrimaryTests/main.swift

@@ -251,6 +251,16 @@ try test("Closure Lifetime") {
     }
 #endif
 
+    do {
+        let c1 = JSClosure { _ in .number(4) }
+        try expectEqual(c1(), .number(4))
+    }
+
+    do {
+        let c1 = JSClosure { _ in fatalError("Crash while closure evaluation") }
+        let error = try expectThrow(try evalClosure.throws(c1)) as! JSValue
+        try expectEqual(error.description, "RuntimeError: unreachable")
+    }
 }
 
 try test("Host Function Registration") {

Sources/JavaScriptKit/FundamentalObjects/JSClosure.swift

@@ -61,7 +61,7 @@ public class JSOneshotClosure: JSObject, JSClosureProtocol {
 /// button.removeEventListener!("click", JSValue.function(eventListenter))
 /// ```
 ///
-public class JSClosure: JSObject, JSClosureProtocol {
+public class JSClosure: JSFunction, JSClosureProtocol {
 
     // Note: Retain the closure object itself also to avoid funcRef conflicts
     fileprivate static var sharedClosures: [JavaScriptHostFuncRef: (object: JSObject, body: ([JSValue]) -> JSValue)] = [:]