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/op.rs b/src/op.rs
index cac1741..6393e00 100644
--- a/src/op.rs
+++ b/src/op.rs
@@ -1,5 +1,5 @@
 ast_enum! {
-    #[derive(Copy)]
+    #[cfg_attr(feature = "clone-impls", derive(Copy))]
     pub enum BinOp {
         /// The `+` operator (addition)
         Add,
@@ -41,7 +41,7 @@
 }
 
 ast_enum! {
-    #[derive(Copy)]
+    #[cfg_attr(feature = "clone-impls", derive(Copy))]
     pub enum UnOp {
         /// The `*` operator for dereferencing
         Deref,