optimize __createBinding (#168)

Reflect microsoft/TypeScript#46997:

When the binding is itself one that was created by `__createBinding`,
re-use its descriptor, which avoids piling multiple levels of getters in
the case of multiple levels of exports.

In addition, reuse a descriptor if the bindings is marked as
non-writable and non-configurable, which makes a getter not
necessary.  (This can be done manually if needed, even though tsc
doesn't do it now.)

Could be considered as a fix for #165 -- first, this PR prevents piling
up multiple layers of getters.  Second, it allows a hack of adding

    if (typeof exports === "object") exports = Object.freeze(exports);

to avoid getters altogether.  (And in the future, tsc could mark `const`
exports as non-writable and non-configurable which would make it
possible to avoid this hack.)

tslib.es6.js

@@ -107,7 +107,11 @@ export function __generator(thisArg, body) {
 
 export var __createBinding = Object.create ? (function(o, m, k, k2) {
     if (k2 === undefined) k2 = k;
-    Object.defineProperty(o, k2, { enumerable: true, get: function() { return m[k]; } });
+    var desc = Object.getOwnPropertyDescriptor(m, k);
+    if (!desc || ("get" in desc ? !m.__esModule : desc.writable || desc.configurable)) {
+        desc = { enumerable: true, get: function() { return m[k]; } };
+    }
+    Object.defineProperty(o, k2, desc);
 }) : (function(o, m, k, k2) {
     if (k2 === undefined) k2 = k;
     o[k2] = m[k];

tslib.js

@@ -153,7 +153,11 @@ var __createBinding;
 
     __createBinding = Object.create ? (function(o, m, k, k2) {
         if (k2 === undefined) k2 = k;
-        Object.defineProperty(o, k2, { enumerable: true, get: function() { return m[k]; } });
+        var desc = Object.getOwnPropertyDescriptor(m, k);
+        if (!desc || ("get" in desc ? !m.__esModule : desc.writable || desc.configurable)) {
+            desc = { enumerable: true, get: function() { return m[k]; } };
+        }
+        Object.defineProperty(o, k2, desc);
     }) : (function(o, m, k, k2) {
         if (k2 === undefined) k2 = k;
         o[k2] = m[k];