src: fix Environment::GetCurrent() resource leak

Environment::GetCurrent(Isolate*) makes a call to
Isolate::GetCurrentContext(), which returns a Local<Context>. Hence,
every call to GetCurrent(Isolate*) should be made within a HandleScope
or EscapableHandleScope.

Also removed excessive calls to GetCurrent().

/cc @bnoordhuis @indutny 
This is unsafe.  `ReturnValue<T>::Set(double)` calls `Set(Number::New(isolate, value))`.