syntax::ext: Make type errors in deriving point to the field itself.

[huonw]

This rearranges the deriving code so that #[deriving] a trait on a field
that doesn't implement that trait will point to the field in question,
e.g.

struct NotEq; // doesn't implement Eq

#[deriving(Eq)]
struct Foo {
    ok: int,
    also_ok: ~str,
    bad: NotEq // error points here.
}

Unfortunately, this means the error is disconnected from the deriving
itself but there's no current way to pass that information through to
rustc except via the spans, at the moment.

Fixes #7724.

[huonw]

Summary: this reuses the spans from the variant and struct_field (ast types) in expand_static_enum_body, summarise_struct, create_struct_pattern and create_enum_pattern to create the span field in the new FieldInfo struct (and the Span parts of StaticFields) which can then be used "downstream" (example) to associate the span of the constructed Exprs with the fields, so any type errors get displayed as if they were caused by that field.

(Sorry for the noise with the span -> trait_span.)