Fix compilation

Something changed in Rust, now Enums are namespaced.

There's still an error I don't know how to fix tho:

```
src/lib/codegen.rs:305:9: 305:10 error: cannot bind by-move into a pattern guard [E0008]
src/lib/codegen.rs:305         t if field_type_wire_type(t) == wire_format::WireTypeFixed32 => Some(4),
                               ^
src/lib/codegen.rs:306:9: 306:10 error: cannot bind by-move into a pattern guard [E0008]
src/lib/codegen.rs:306         t if field_type_wire_type(t) == wire_format::WireTypeFixed64 => Some(8),
```