Move all #[derive] impls behind Cargo feature gates

This commit moves all #[derive] annotations behind Cargo feature gates to add
the ability to strip them all out entirely. The `Clone` and `Copy` impls
continue to be enabled by default as they tend to be mega useful but other
equality/hash/debug impls are all default behind the `extra-impls` gate.

This commit, on my computer, has the following timings:

| features                      | before  | after
|-------------------------------|---------|------
| default                       | 3.67    | 2.96
| *none*                        | 1.78    | 0.49
| {printing, parsing}           | 3.71    | 2.57
| default + {full}              | 8.50    | 6.31
| {full}                        | 3.53    | 0.70
| {full, printing, parsing}     | 8.10    | 5.29

Closes #143
diff --git a/src/data.rs b/src/data.rs
index d01d9e5..3916b25 100644
--- a/src/data.rs
+++ b/src/data.rs
@@ -247,7 +247,6 @@
 mod printing {
     use super::*;
     use quote::{Tokens, ToTokens};
-    use ty::PathParameters;
 
     impl ToTokens for Variant {
         fn to_tokens(&self, tokens: &mut Tokens) {
@@ -312,7 +311,7 @@
                     if !path.global &&
                        path.segments.len() == 1 &&
                        (path.segments[0].ident == "self" || path.segments[0].ident == "super") &&
-                       path.segments[0].parameters == PathParameters::none() {
+                       path.segments[0].parameters.is_empty() {
 
                         // Don't emit preceding `in` if path is `self` or `super`
                     } else {