feat(codegen): Inhibit generic bounds if skip_serializing

The generated code for a struct like:

    struct Test<A, B, C> {
        a: X<A>
        #[serde(skip_serializing)]
        b: B
        #[serde(serialize_with="...")]
        c: C
    }

Used to be:

    impl<A, B, C> Serialize for Test<A, B, C>
        where A: Serialize,
              B: Serialize,
              C: Serialize,
    { ... }

Now it is:

    impl<A, B, C> Serialize for Test<A, B, C>
        where X<A>: Serialize,
    { ... }

Both `skip_serializing` and `serialize_with` mean the type does not need to
implement `Serialize`.

serde_codegen/src/attr.rs

@@ -382,7 +382,7 @@ fn get_renames(cx: &ExtCtxt,
     Ok((ser_name, de_name))
 }
 
-fn get_serde_meta_items(attr: &ast::Attribute) -> Option<&[P<ast::MetaItem>]> {
+pub fn get_serde_meta_items(attr: &ast::Attribute) -> Option<&[P<ast::MetaItem>]> {
     match attr.node.value.node {
         ast::MetaItemKind::List(ref name, ref items) if name == &"serde" => {
             attr::mark_used(&attr);