fix: do not use external string as keys (nodejs#42)

blink can not recognize external strings from Node, and it will crash
when encountered one, for example executing `window.string_decoder`.

Note that the values are still using external strings, but making them
normal string may have negative affects on performance, and it is very
unlikely they would be passed to blink.

tools/js2c.py

@@ -232,7 +232,10 @@ def ReadMacros(lines):
 
 INITIALIZER = """\
 CHECK(target->Set(env->context(),
-                  {key}.ToStringChecked(env->isolate()),
+                  v8::String::NewFromUtf8(env->isolate(),
+                                          {key}.data(),
+                                          v8::NewStringType::kNormal,
+                                          arraysize(raw_{key})).ToLocalChecked(),
                   {value}.ToStringChecked(env->isolate())).FromJust());
 """