Add source annotation to express explicit derives for a type.

This allows for explicit selection of which traits are derived for a
type, e.g. deriving custom traits on particular types that need it.

Example usage:

```C++
/// <div rustbindgen derive="Clone"></div>
/// <div rustbindgen derive="MyDerivableTrait"></div>
struct foo { ...  };
```

generates into

```Rust
 #[derive(Clone,MyDerivableTrait)]
 struct foo { ... }
```

1 files changed, 2 insertions, 0 deletions

diff --git a/src/codegen/mod.rs b/src/codegen/mod.rs
index 073745b1..9405f11b 100644
--- a/src/codegen/mod.rs
+++ b/src/codegen/mod.rs
@@ -1764,6 +1764,8 @@ impl CodeGenerator for CompInfo {
             derives.push("Eq");
         }
 
+        derives.extend(item.annotations().derives().iter().map(String::as_str));
+
         if !derives.is_empty() {
             attributes.push(attributes::derives(&derives))
         }