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. 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 passing either a pointer to the
   struct-containing-a-pointer or the struct itself.
 * Unit enums as enums with each type copied over and conversion
   functions,
 * Non-unit enums the same way as structs.

Author: TheBlueMatt

c-bindings-gen/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "c-bindings-gen"
+version = "0.0.1"
+authors = ["Matt Corallo"]
+edition = "2018"
+
+[dependencies]
+syn = { version = "1", features = ["full", "extra-traits"] }
+proc-macro2 = "1"
+
+# We're not in the workspace as we're just a binary code generator:
+[workspace]