More Rust FFI adventures

We can now convert Rust Errors to C++ exceptions. At the Rust->C++ FFI
boundary, Result<T, Error> will cause Error to be converted to and
thrown as a C++ exception.

nix-rust/src/error.rs

@@ -1,5 +1,30 @@
 #[derive(Debug)]
 pub enum Error {
+    IOError(std::io::Error),
     Misc(String),
-    Foreign(libc::c_void), // == std::exception_ptr
+    Foreign(CppException),
+}
+
+impl From<std::io::Error> for Error {
+    fn from(err: std::io::Error) -> Self {
+        Error::IOError(err)
+    }
+}
+
+impl From<Error> for CppException {
+    fn from(err: Error) -> Self {
+        match err {
+            Error::Foreign(ex) => ex,
+            Error::Misc(s) => unsafe { make_error(&s) },
+            Error::IOError(err) => unsafe { make_error(&err.to_string()) },
+        }
+    }
+}
+
+#[repr(C)]
+#[derive(Debug)]
+pub struct CppException(*const libc::c_void); // == std::exception_ptr*
+
+extern "C" {
+    fn make_error(s: &str) -> CppException;
 }