Extend #[deriving] to allow options to be specified

[huonw]

This would make deriving significantly more flexible, e.g. for #3740:

#[deriving(Encodable(encoder="RustcEncoder"))]
struct Foo { ... }

And for specifying an order of fields to sort by:

#[deriving(Ord(bar, baz))]
struct Foo { baz: uint, bar: uint }

[emberian]

Nominating for feature complete

[huonw]

@cmr points out that being able to specify which fields are relevant for Eq would be neat too.

[catamorphism]

Needs discussion, de-nominating for now.

[emberian]

Visiting for triage... this was rejected once, in code form. Closing :(