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/mac.rs b/src/mac.rs
index e6132e4..dbed4eb 100644
--- a/src/mac.rs
+++ b/src/mac.rs
@@ -89,7 +89,7 @@
 }
 
 ast_enum! {
-    #[derive(Copy)]
+    #[cfg_attr(feature = "clone-impls", derive(Copy))]
     pub enum BinOpToken {
         Plus,
         Minus,
@@ -106,7 +106,7 @@
 
 ast_enum! {
     /// A delimiter token
-    #[derive(Copy)]
+    #[cfg_attr(feature = "clone-impls", derive(Copy))]
     pub enum DelimToken {
         /// A round parenthesis: `(` or `)`
         Paren,
@@ -371,8 +371,8 @@
                 Token::OrOr => tokens.append("||"),
                 Token::Not => tokens.append("!"),
                 Token::Tilde => tokens.append("~"),
-                Token::BinOp(binop) => tokens.append(binop.op()),
-                Token::BinOpEq(binop) => tokens.append(binop.assign_op()),
+                Token::BinOp(ref binop) => tokens.append(binop.op()),
+                Token::BinOpEq(ref binop) => tokens.append(binop.assign_op()),
                 Token::At => tokens.append("@"),
                 Token::Dot => tokens.append("."),
                 Token::DotDot => tokens.append(".."),