Support compiled code through shared libraries

[frankmcsherry]

Timely and differential dataflow have primitive support for loading shared libraries that can build dataflows. It is almost certainly undefined behavior in many ways, but it is possible that it is not undefined in that many ways.

The repo is https://github.com/TimelyDataflow/differential-dataflow/tree/master/server which may need to be tidied up.

The classes of issues include, but may not be limited to:

1. shared libraries reportedly should be linked against the standard library dynamically, so that they do not load their own instance,
2. as we intend to pass allocations across the FFI boundary we probably want the same allocator or some magic to de-allocate correctly,
3. types that cross the boundary are likely not repr anything, which would be deeply problematic except that Rust's linking requires consistency in layout.

I'm sure there are other issues as well.

It may be that this issue involves a bit of advocacy with the Rust community, to try and get some attention paid to dynamic linking of Rust on Rust code. It may also be a terrible idea and we shouldn't do it, or perhaps we should use Graal, or perhaps ...