Mark some derived methods as #[inline].

[huonw]

ToStr, Encodable and Decodable are not marked as such, since they're
already expensive, and lead to large methods, so inlining will bloat the
metadata & the binaries.

This means that something like

#[deriving(Eq)]
struct A { x: int }

creates an instance like

#[doc = "Automatically derived."]
impl ::std::cmp::Eq for A {
    #[inline]
    fn eq(&self, __arg_0: &A) -> ::bool {
        match *__arg_0 {
            A{x: ref __self_1_0} =>
            match *self {
                A{x: ref __self_0_0} => true && __self_0_0.eq(__self_1_0)
            }
        }
    }
    #[inline]
    fn ne(&self, __arg_0: &A) -> ::bool {
        match *__arg_0 {
            A{x: ref __self_1_0} =>
            match *self {
                A{x: ref __self_0_0} => false || __self_0_0.ne(__self_1_0)
            }
        }
    }
}

(The change being the #[inline] attributes.)