Automatic generation of FFI wrappers

[andrewvyrros]

Currently, when adding a component, we need to manually create the FFI wrapper function and its associated bootstrap.json metadata. It'd be fantastic if this could be done automatically from the underlying Rust function in a build script. But this is hard!

There are a couple of ways that this might work:

• Proc macros invoked directly on the functions in opendp to generate the wrappers. This has the downside of introducing (generated) unsafe code into core.
• A build script in opendp-ffi that parses the functions in opendp to generate the wrappers. This would require a little more plumbing, but the code would be the same (presumably using the syn crate), and it avoids the unsafe issue.

One of the main stumbling blocks is wrappers requiring complex dispatch on type. The macros in dispatch.rs only handle one level of indirection, so this means implementing multiple nesting monomorphize() functions.

An intermediate step towards this could be #42.

[raprasad]

Large project w/ low payoff in regard to use cases.

(Polars will significantly reduce FFI code which may make this more plausible in the future.)