Add tool to read a Rust crate and generate C-compatible wrappers

Add tool to read a Rust crate and generate C-compatible wrappers

In general, it maps:
 * Traits to a struct with a void* and a list of function pointers,
   emulating what the compiler will do for a dyn trait anyway,
 * Structs as a struct with a single opaque pointer to the
   underlying type and a flag to indicate ownership. While this is
   a bit less effecient than just a direct pointer, it neatly lets
   us expose in the public interface the concept of ownership by
   setting a flag in the generated struct.
 * Unit enums as enums with each type copied over and conversion
   functions,
 * Non-unit enums have each field converted back and forth with a
   type flag and a union across all the C-mapped fields.