Allow direct file access

[jqmp]

Right now, Bionic entity functions can only accept and return in-memory objects. However, in some cases we want to operate directly on files. (For example, we may want to call out to command line tools.) We can sort of replicate this by generating files and passing around paths, but this doesn't integrate with our serialization and caching systems. It would be better if we could indicate that a function's input and/or output correspond to the serialized version of an entity. The second example of @bn.arg in https://github.com/square/bionic/issues/46 is a good example of how this might work.