convience methods for skipping library deployment

deploying libraries can take a long time. One can easily skip this stage if we make it possible to save the deployed library state and pass it to `dapp test`:

1) Write a new little dapp helper script which is the `dapp --library-addresses` above
2) Make `dapp build` and `dapp --make-library-state` use this helper script instead
3) Allow a flag `--state` to be passed to `dapp test` (probably along with env var `DAPP_TEST_STATE`), which would override the default behaviour of the library deployment that happens automatically

Now if you want to cache this annoying 45s process that happens everytime, you would put:

```
export DAPP_LIBRARIES=$(dapp --library-addresses)
export DAPP_TEST_STATE=$(dapp --make-library-state)

```

in your makefile