ffi: don't use empty enums for opaque types

The nomicon doesn't like this. From the bottom of

https://doc.rust-lang.org/nomicon/ffi.html

"Notice that it is a really bad idea to use an empty enum as FFI type. The
compiler relies on empty enums being uninhabited, so handling values of
type &Empty is a huge footgun and can lead to buggy program behavior (by
triggering undefined behavior)."

They give an example that uses a phantom to also remove Send, Sync, and
Unpin. I did not do this. I think all these traits are fine, because the
type has no self-references and is never mutated by the C code except
when it is freed, and then only through a `*mut` pointer.

Author: apoelstra

simplicity-sys/src/c_jets/c_env/elements.rs

@@ -130,8 +130,10 @@ pub struct CRawTapEnv {
     pub branch_len: u8,
 }
 
-#[derive(Debug)]
-pub enum CTransaction {}
+#[repr(C)]
+pub struct CTransaction {
+    _data: (),
+}
 
 #[derive(Debug)]
 #[repr(C)]
@@ -143,8 +145,10 @@ pub struct CTxEnv {
     ix: c_uint_fast32_t,
 }
 
-#[derive(Debug)]
-pub enum CTapEnv {}
+#[repr(C)]
+pub struct CTapEnv {
+    _data: (),
+}
 
 extern "C" {
     #[link_name = "rustsimplicity_0_5_c_sizeof_rawElementsBuffer"]