Remove Result alias from API of cxx-gen library

This is unnecessary when we don't expect downstream code to be commonly
passing around cxx_gen::Result<T>.
diff --git a/gen/src/error.rs b/gen/src/error.rs
index 9a55243..70dcb37 100644
--- a/gen/src/error.rs
+++ b/gen/src/error.rs
@@ -11,7 +11,7 @@
 use std::path::Path;
 use std::process;
 
-pub type Result<T, E = Error> = std::result::Result<T, E>;
+pub(crate) type Result<T, E = Error> = std::result::Result<T, E>;
 
 #[derive(Debug)]
 pub enum Error {