Add `fromBuilder`

This is actually more of a question than a pull-request.

I'm experimenting between working directly with `ByteString` vs using `Builder`s. By using `Builder`s I get the benefit of automatic buffering but I don't know if introducing lazy bytestrings in the process will make it worthwhile.

Do you think this, in the general case, is better than working with `ByteString`s directly and introduce another custom pipe component to deal with buffering?

If you think the latter option is in most cases preferable, don't hesitate in rejecting this pull request.

Notice that I had to bump. If you want to support lower versions of `bytestring`, I could introduce a CPP pragma.