Use a primitive field mapping from symbol to field class

Generate “prettier” message code that uses symbols for all the
primitive types in the definition.

```ruby
 optional ::Protobuf::Field::StringField, :bar, 1 end

 optional :string, :foo, 1 end
```

You can mostly ignore the Rakefile and *.pb.rb file changes, the relevant changes are in `lib/protobuf/field.rb` and `lib/protobuf/generators/field_generator.rb`.