Sudo RPC library

To do local RPC, we need to use midl to generate function bindings for our RPC interface, which is defined in sudo_rpc.idl. midl expects implementations and callbacks via C functions which we can do in Rust, but there's one problem: Error handling on the client side occurs with structed exceptions (SEH). Those cannot be easily replicated in pure Rust and so the client side calls are all wrapped in C functions.

Changes here go as follows:

• Change the interface in sudo_rpc.idl
• Write a client-side wrapper in RpcClient.c in the style of the other ones
• Implement a client-side wrapper Rust in rpc_bindings_client.rs
• Implement the server-side part in rpc_bindings_server.rs

Be careful about the function definitions. At the moment, there are no checks in place that ensure that the Rust code matches the C code or the .idl file.