save a huge number of bytes in `require('util')`

There is a reference to `Buffer` in util.js, but only to check instances in `util.isBuffer()`. By checking property names instead, we can drastically reduce the number of bytes in util.js because Buffer pulls in a lot of other machinery that is not going to be necessary most of the time.

before:

```
$ browserify <(echo "require('util')") | wc -c
87248
```

after:
```
$ browserify <(echo "require('util')") | wc -c
21176
```