Change serde_path to crate

Also changed the generated code to have at least one thing refer to the
path directly, rather than via `use` -- This shows that the impl *can*
work without `use`, but doesn't actually do all the work to remove the
`use` lines unless we decide we need this feature to work on the 2015
edition

test_suite/tests/test_serde_path.rs

@@ -1,7 +1,7 @@
 #[test]
 fn test_gen_custom_serde() {
     #[derive(serde::Serialize, serde::Deserialize)]
-    #[serde(serde_path = "fake_serde")]
+    #[serde(crate = "fake_serde")]
     struct Foo;
 
     // Would be overlapping if serde::Serialize were implemented